stop using `FnCtxt` outside of hir typeck
the requirements between **hir typeck**™, and **not hir typeck**™ are different enough to warrant a full split. with this PR `FnCtxt` is now only used for hir typeck (and for one clippy lint which seems to be emulating hir typeck).
Once this PR has landed I intend to move `FnCtxt` into a new crate. This should also allow some further general improvements here.
r? rust-lang/types
23e21bdd25