Objective-C diagnostics: isObjCNSObjectType should check through AttributedType.

For the following example:
typedef __attribute__((NSObject)) CGColorRef ColorAttrRef;
@property (strong, nullable) ColorAttrRef color;
The property type should be ObjC NSObject type and the compiler should not emit
error: property with 'retain (or strong)' attribute must be of object type

rdar://problem/27747154

llvm-svn: 278742