[ObjC] Make attribute 'objc_protocol_requires_explicit_implementation' behave...

[ObjC] Make attribute 'objc_protocol_requires_explicit_implementation' behave correctly with default property synthesis.

In particular, if we see an @property within the @interface of a class
conforming to a protocol with this attribute, we treat that as
if the implementation were available, per the rules of default
property synthesis.

llvm-svn: 201911