coverage: Check for `async fn` explicitly, without needing a heuristic
The old code used a heuristic to detect async functions and adjust their coverage spans to produce better output. But there's no need to resort to a heuristic when we can just look back at the original definition and check whether the current function is actually an `async fn`.
In addition to being generally nicer, this also gets rid of the one piece of code that specifically cares about `CoverageSpan::is_closure` representing an actual closure. All remaining code that inspects that field just uses it as an indication that the span is a hole that should be carved out of other spans, and then discarded.
That opens up the possibility of introducing other kinds of “hole” spans, e.g. for nested functions/types/macros, and having them all behave uniformly.
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`@rustbot` label +A-code-coverage
5906d8f0cf