coverage: Extract hole spans from HIR instead of MIR
This makes it possible to treat more kinds of nested item/code as holes, instead of being restricted to closures.
(It also potentially opens up the possibility of using HIR holes to modify branch or MC/DC spans, though we currently don't actually do this.)
Thus, this new implementation treats the following as holes:
- Closures (as before, including `async` and coroutines)
- All nested items
- Inline `const` (because why not)
This gives more accurate coverage reports, because lines occupied by holes don't show the execution count from the enclosing function.
Fixes#126626.
bors
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