Auto merge of #158974 - bal-e:macro-parsing-dfs-prep, r=nnethercote

Preparatory changes for macro parsing BFS->DFS



This PR contains some miscellaneous commits I accumulated while working on the BFS->DFS change. Their goal is to simplify the code and clarifying existing behavior. It is a conceptual follow-up to rust-lang/rust#158577. High-level overview:

- Adds context about the current match arm to `Tracker` through the new `Tracker::prepare()`, so that the `WhichMatcher` parameter is available implicitly. In a later PR, this will be used to reference `MatcherLoc`s by index.

- Reformulates matching failures to work more like ambiguity errors wrt. `Tracker`; `Tracker::build_failure()` (which would build a failure consumed by `Tracker::after_arm()`) becomes `Tracker::failure()` which eagerly processes the error. This removes the need for `ParseResult::Failure` to store any data at all. This relies on the match arm context provided by `Tracker::prepare()`.

- There is a subtle edge case involving `token::Eof` and non-terminal parsing; `Parser::nonterminal_may_begin_with()` would sometimes return `true` for `token::Eof`, even though the non-terminal parse would never be attempted. `TtParser` did not check `bb_mps` when handling `token::Eof`, so non-terminal parses at EOF were being silently dropped. I changed `Parser::nonterminal_may_begin_with()` to always return `false` for `token::Eof`, making this behavior much more visible. I added a test case to make sure meta-variables return the same error uniformly when parsed against EOF.

Contains rust-lang/rust#158894, which should be merged soon (after which I'll rebase onto `main`).

Best reviewed commit-by-commit.

r? @nnethercote