Rollup merge of #153018 - WaffleLapkin:must_use_improvements, r=jdonszelmann

`unused_must_use` lint improvements

The main purpose of this is to allow clippy to use `is_ty_must_use` from rustc, rather than making their own (outdated) copy. This also adds changes from rust-lang/rust#148214 under a function flag, so clippy can start linting with the new semantics in mind, while rustc doesn't yet (see rust-lang/rust#148214 (comment)).

I've also done some refactoring while I was at it.

r? @jdonszelmann

Author: JonathanBrouwer