coverage: Count await when the Future is immediately ready
Currently `await` is only counted towards coverage if the containing
function is suspended and resumed at least once. This is because it
expands to code which contains a branch on the discriminant of `Poll`.
By treating it like a branching macro (e.g. `assert!`), these
implementation details will be hidden from the coverage results.
I added a test to ensure the fix works in simple cases, but the heuristic of picking only the first await-related covspan might be unreliable. I plan on testing more thoroughly with a real codebase over the next couple of weeks.
closes#98712
11d5614a74