[JuliaLowering] Replace Tuple `.source` with NodeId `.macro_source` (#61597)

## Macro expansion performance issue

Noted in #61425, but here's a recap: Currently, we represent
multi-provenance (a
tree originating from source text and >=1 macro expansions) by setting
its
nodes' `.source` attributes to a tuple of NodeIds. This is slow: when we
macroexpand, we need to copy, unpack, and repack the tuple associated
with every
subtree coming from the expansion. It also duplicates information we
rarely
ever need, since this is only used for debuginfo production and certain
pretty-printing. We're usually only interested in the first item in the
tuple.

## Changes

### `.macro_source`

This change represents the same information (tuple elements 2:end) with
a single
NodeId `.macro_source` on multiple nodes in a tree's provenance. This
fixes the
performance issue noted in the past, but also improves serializability
of
SyntaxTree if we end up putting them on disk.

This representation is similar to lowered `DebugInfo`, where `st.source`
is
`di.linetable` and `st.macro_source` is one of `di.edges`.

### Provenance API improvements

I needed to rewrite `flattened_provenance` and friends to use
`.macro_source`,
so I've also written down what these functions should do.  Some
reverse-engineering was needed, and some functions have changed their
behaviour
slightly, so @aviatesk let me know if there's some functionality that's
missing.
`flattened_provenance` should be the same.

### `copy_ast`, `copy_node`, etc.

Cleaned up given that copying needs to have special behaviour with
`.source` and
`.macro_source`. `mktree` now copies a subtree for lowering purposes
(extends
provenance, recurses on children) and `copy_ast` (actually copies
`.source` and
`.macro_source`) is restricted to the foreign-graph case.

### Benchmarks

Script and GC-time disclaimer is the same as in #61425. Improvement
depends on
how macro-heavy the code is.

<details>
<summary>
Before
</summary>

```
julia> include("jlbench.jl"); @allocated main(["./test/char.jl"])
File: ./test/char.jl
Module: Main
Number of top-level expressions: 40
Number of runs: 10

1...2...3...4...5...6...7...8...9...10...
Best times (over 10 runs):
  expand_forms_1:              43.383 ms
  expand_forms_2:              30.849 ms
  resolve_scopes:              23.396 ms
  convert_closures:            29.841 ms
  linearize_ir:                34.429 ms
  to_lowered_expr:             46.547 ms

  Total:                       208.445 ms
  Total(flisp):                248.6 ms

Percentage breakdown:
  expand_forms_1:              20.8%
  expand_forms_2:              14.8%
  resolve_scopes:              11.2%
  convert_closures:            14.3%
  linearize_ir:                16.5%
  to_lowered_expr:             22.3%
1964449696

julia> include("jlbench.jl"); @allocated main(["./test/atomics.jl"])
File: ./test/atomics.jl
Module: Main
Number of top-level expressions: 392
Number of runs: 10

1...2...3...4...5...6...7...8...9...10...
Best times (over 10 runs):
  expand_forms_1:              112.906 ms
  expand_forms_2:              90.099 ms
  resolve_scopes:              55.319 ms
  convert_closures:            18.995 ms
  linearize_ir:                83.808 ms
  to_lowered_expr:             83.458 ms

  Total:                       444.585 ms
  Total(flisp):                378.801 ms

Percentage breakdown:
  expand_forms_1:              25.4%
  expand_forms_2:              20.3%
  resolve_scopes:              12.4%
  convert_closures:            4.3%
  linearize_ir:                18.9%
  to_lowered_expr:             18.8%
3792501984

julia> include("jlbench.jl"); @allocated main(["./test/numbers.jl"])
File: ./test/numbers.jl
Module: Main
Number of top-level expressions: 416
Number of runs: 10

1...2...3...4...5...6...7...8...9...10...
Best times (over 10 runs):
  expand_forms_1:              777.836 ms
  expand_forms_2:              558.305 ms
  resolve_scopes:              378.443 ms
  convert_closures:            211.51 ms
  linearize_ir:                592.592 ms
  to_lowered_expr:             736.205 ms

  Total:                       3254.891 ms
  Total(flisp):                2588.61 ms

Percentage breakdown:
  expand_forms_1:              23.9%
  expand_forms_2:              17.2%
  resolve_scopes:              11.6%
  convert_closures:            6.5%
  linearize_ir:                18.2%
  to_lowered_expr:             22.6%
20309895520
```
</details>

<details>
<summary>
After
</summary>

```
julia> include("jlbench.jl"); @allocated main(["./test/char.jl"])
File: ./test/char.jl
Module: Main
Number of top-level expressions: 40
Number of runs: 10

1...2...3...4...5...6...7...8...9...10...
Best times (over 10 runs):
  expand_forms_1:              28.689 ms
  expand_forms_2:              32.89 ms
  resolve_scopes:              22.794 ms
  convert_closures:            18.304 ms
  linearize_ir:                37.902 ms
  to_lowered_expr:             26.57 ms

  Total:                       167.149 ms
  Total(flisp):                243.393 ms

Percentage breakdown:
  expand_forms_1:              17.2%
  expand_forms_2:              19.7%
  resolve_scopes:              13.6%
  convert_closures:            11.0%
  linearize_ir:                22.7%
  to_lowered_expr:             15.9%
1696645648

julia> include("jlbench.jl"); @allocated main(["./test/atomics.jl"])
File: ./test/atomics.jl
Module: Main
Number of top-level expressions: 392
Number of runs: 10

1...2...3...4...5...6...7...8...9...10...
Best times (over 10 runs):
  expand_forms_1:              73.671 ms
  expand_forms_2:              86.262 ms
  resolve_scopes:              52.325 ms
  convert_closures:            19.826 ms
  linearize_ir:                81.106 ms
  to_lowered_expr:             58.467 ms

  Total:                       371.658 ms
  Total(flisp):                375.771 ms

Percentage breakdown:
  expand_forms_1:              19.8%
  expand_forms_2:              23.2%
  resolve_scopes:              14.1%
  convert_closures:            5.3%
  linearize_ir:                21.8%
  to_lowered_expr:             15.7%
3688971328

julia> include("jlbench.jl"); @allocated main(["./test/numbers.jl"])
File: ./test/numbers.jl
Module: Main
Number of top-level expressions: 416
Number of runs: 10

1...2...3...4...5...6...7...8...9...10...
Best times (over 10 runs):
  expand_forms_1:              585.484 ms
  expand_forms_2:              571.274 ms
  resolve_scopes:              364.441 ms
  convert_closures:            142.187 ms
  linearize_ir:                615.537 ms
  to_lowered_expr:             461.66 ms

  Total:                       2740.583 ms
  Total(flisp):                2560.689 ms

Percentage breakdown:
  expand_forms_1:              21.4%
  expand_forms_2:              20.8%
  resolve_scopes:              13.3%
  convert_closures:            5.2%
  linearize_ir:                22.5%
  to_lowered_expr:             16.8%
20738718400
```
</details>

If I really cheat (minimum time over 10 runs like above, `const DEBUG =
false`
in JuliaLowering), it is reliably faster than flisp now :). There's
still much
more work to do in the practical case; memory use in particular needs
improvement.