`InstrumentCoverage` requires information from HIR that might need to be available via MIR

[richkadel]

Coverage requires the spans for the FnSig and hir::Body.

It also uses the hir::Body to compute a function hash, required by LLVM intrinsic instrprof.coverage.

According to this code review comment:

#79109 (comment)

MIR strives to be a self-contained IR which means that operations on it should only use the information in MIR

Consider improving MIR to provide the information needed, to avoid going back to HIR for it.

[richkadel]

Just noting for clarification: There are about a dozen cases where HIR data is accessed to generate the MIR (based on a simple regex search hir.*Body), most of which are not related to coverage.

I'm not sure that the A-code-coverage label is necessary, but, regardless, IMO, this issue should not impact stabilization of Rust Coverage.

cc: @tmandry @wesleywiser