Objective-C. Tweak diagnosing properties that are not auto-synthesized.

Do not warn when property declared in class's protocol will be auto-synthesized
by its uper class implementation because super class has also declared this
property while this class has not. Continue to warn if current class
has declared the property also (because this declaration will not result
in a 2nd synthesis).
rdar://18152478

llvm-svn: 216753