Avoid misleading return-type note for foreign Fn callees

[qaijuang]

Fixes #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 #125325 was fixed in #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 #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.

[qaijuang]

cc @rustily-indecisive

[rustbot]

r? @wesleywiser

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