Consider moving instrument-coverage-specific behavior in rustc_typeck to backend code

[richkadel]

See review comment #83307 (comment) suggesting we may want to move the -Z instrument-coverage-specific behavior from rustc_typeck to a place more relevant to LLVM and/or codegen.

rustc_typeck appears to be the most central place to apply this fix. Is there a better place, or if not, can we add one?