coverage: Improve handling of function/closure spans

[Zalathar]

This is a combination of some loosely-related changes that touch the same code:

1. Make unexpansion of closure bodies more precise, by unexpanding back to the context of the closure declaration, instead of unexpanding all the way back to the top-level context. This preserves the way we handle async desugaring and closures containing a single bang-macro, while also giving better results for closures defined in macros.
2. Skip the normal span-refinement code when dealing with the trivial outer part of an async function.
3. Be more explicit about the fact that fn_sig_span has been extended to the start of the function body, and is not necessarily present.

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@rustbot label +A-code-coverage

[rustbot]

r? @TaKO8Ki

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[oli-obk]

@bors r+ rollup

r? @oli-obk

[bors]

:pushpin: Commit 23fb8946b0372c1340a083e75e3e24c56352fc31 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

:evergreen_tree: The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

[Zalathar]

This was excluded from rollup #120620 because it has an incidental conflict with #120566 in some test headers.

Since that rollup will hopefully land relatively soon, I’ll just rebase this afterwards.

[bors]

:umbrella: The latest upstream changes (presumably #120620) made this pull request unmergeable. Please resolve the merge conflicts.

[Zalathar]

Rebased to resolve trivial conflict in a test file; no functional changes.

[Zalathar]

@rustbot ready

[oli-obk]

@bors r+

[bors]

:pushpin: Commit a246b6be1dcd4a8ea0ac468921af7e8022686849 has been approved by oli-obk

It is now in the queue for this repository.