coverage: Use original HIR info for synthetic by-move coroutine bodies

[qaijuang]

Synthetic by-move coroutine bodies created for async closures don't have useful HIR of their own. Coverage was falling back to the parent async closure for HIR info, which means the executed AsyncFnOnce body could inherit hole spans from the wrong body and report the user-written closure body as uncovered.

This PR uses the synthetic body's coroutine type to recover the original coroutine body def-id, then extracts HIR info from that body instead. That keeps the coverage spans tied to the body the synthetic MIR was cloned from.

Fixes #151135.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in coverage tests.

cc @Zalathar

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[qaijuang]

my git is acting weird, i'll reopen this as a new PR