Rollup merge of #120569 - Zalathar:fn-sig, r=oli-obk

coverage: Improve handling of function/closure spans

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

0 files changed, 0 insertions, 0 deletions