`Core.DebugInfo`: expand C API, mostly to handle byte-precise information (#61817)

This change adds a basic C API for reading `jl_debuginfo_t`, both in its
current form and with byte-precise provenance to be generated by
JuliaLowering.

```
(debuginfo, pc) -> (firstbyte, lastbyte)
(debuginfo, byte) -> (line, column)
(debuginfo, pc) -> (firstline, firstcolumn)
debuginfo -> first_line_of_entire_codeinfo
```

Note that the byte-precise provenance is not yet produced (and I've
marked those branches unreachable). I'm making this change first so I
can use them to try and resolve issues in #61699 and #53925. (Namely,
I'm going to try and compute inlined frames from julia debuginfo
rather than DWARF.)

## Implementation of byte-precise info storage

The API shouldn't depend on how we do this, but I'll describe it here
for reference.

The simplicity of our current `Core.DebugInfo` (ref. #52415) is thanks
to simple line numbers looking the same as indices into the next 
debuginfo, so they can be stored in the same way. Unfortunately, 
byte-precise information breaks this:
from an index, we want to be able to know two byte positions, what lines
they fall on, and how far from the beginning of those lines they are.

I intend to store this info by putting a string in `linetable` like we
do for `di.codelocs`. The compression format is described in julia.h, but
only a couple points are relevant here:
- Like `di.codelocs` today, I made a reasonable attempt to compress it
  (so we don't blow up the sysimg) without trying to be too clever.
- Position lookup for a single IR statement can still be done without
  decompressing everything.

I've already implemented the compression/decompression code (in
JuliaLowering), but it's not part of this PR. This format might be torn
out when we start trying to store AST provenance.

---------

Co-authored-by: Cody Tapscott <topolarity@tapscott.me>