Avoid misleading return-type note for foreign `Fn` callees
Fixesrust-lang/rust#155727.
The issue occurred because the code that emitted the `Fn`/`FnMut` suggestion only avoided the return-type fallback when it could point at a local callee argument. The local case from rust-lang/rust#125325 was fixed in rust-lang/rust#126226, but nested cross-crate cases could still suggest changing the enclosing function's return type even though the relevant `Fn` requirement came from the callee argument.
This is also the broader diagnostic shape discussed in rust-lang/rust#119985.
This PR checks the instantiated callee predicates for an exact `Fn` bound
on the closure argument, and extends `wrong-closure-arg-suggestion-125325` with cross-crate function and method cases for that path.
Jonathan Brouwer
cb4bb14237