Implement new default property synthesis rules. Essentially, no longer

Implement new default property synthesis rules. Essentially, no longer
user directive is needed to force a property implementation.
It is decided based on those propeties which are declared in
the class (or in its protocols) but not those which must be
default implemented by one of its super classes. Implements radar 7923851.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103787 91177308-0d34-0410-b5e6-96231b3b80d8