Merge together several helpers and code patterns for “diagnose/fix-it/invalidate bad attribute” into helper functions in TypeChecker.h.
This requires minor test changes in some places where we’re testing ObjC interop without importing Foundation; it’s otherwise NFC.
A bunch of tests that technically require Objective-C interop were
not labeled as such, and worked in the absence of Objective-C interop
due to bugs in the type checker.
