[objc_direct] fix uniquing when re-declaring a readwrite-direct property

ObjCMethodDecl::getCanonicalDecl() for re-declared readwrite properties,
only looks in the ObjCInterface for the declaration of the setter
method, which it won't find.

When the method is a property accessor, we must look in extensions for a
possible redeclaration.

Radar-Id: rdar://problem/57991337

Differential Revision: https://reviews.llvm.org/D71588