Sema: allow applying @objc @implementation on extensions to internally imported types

[nkcsgexi]

Extend the existing @c/@objc @implementation hidden-dependency exemption to cover the extension's extended-type and where-clause checks

rdar://178750930

[nkcsgexi]

@swift-ci please smoke test

[tshortli]

This looks generally reasonable, but I think it's missing some test coverage for like adding protocol conformances to the extension. What happens if you, for instance, add some public Swift-only conformance like CustomStringConvertible to @objc @implementation extension ObjCImplClass? Would the compiler try to print that conformance in the .swiftinterface for the module? If so, we can't allow that reference which I think this change would.

[nkcsgexi]

Great catch! Let me add more test coverage and ensure this new exception doesn't regress textual interface validity.