The currying behavior of method references completely breaks in the face of `inout` semantics, even moreso with exclusivity enforcement, but we failed to diagnose these references in Swift 4 and previous versions. Raise a compatibility warning when these references are found in Swift 4 code, or error in Swift 5 and later. Simplify the partial application logic here slightly too now that standalone functions do not allow currying. Addresses rdar://problem/41361334 | SR-8074.
