Auto merge of #117827 - Zalathar:bogus-macro-name-span, r=davidtwco

coverage: Avoid creating malformed macro name spans

This is a workaround for #117788. It detects a particular scenario where we would create malformed coverage spans that might cause `llvm-cov` to immediately exit with an error, preventing the user from processing coverage reports.

The patch has been kept as simple as possible so that it's trivial to backport to beta (or stable) if desired.

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The `maybe_push_macro_name_span` method is trying to detect macro invocations, so that it can split a span into two parts just after the `!` of the invocation.

Under some circumstances (probably involving nested macros), it gets confused and produces a span that is larger than the original span, and possibly extends outside its enclosing function and even into an adjacent file.

In extreme cases, that can result in malformed coverage mappings that cause `llvm-cov` to fail. For now, we at least want to detect these egregious cases and avoid them, so that coverage reports can still be produced.

Author: bors