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

See review comment https://github.com/rust-lang/rust/pull/83307#discussion_r600011365 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?