Add synthesized property methods to protocols and categories in Sema.

Add synthesized property methods to protocols and categories in Sema.

 - It is not clear that this is the right approach, but this is at
   least consistent with how interfaces are handled.

 - This means NeXT now emits the correct metadata for properties in
   protocols.

 - This currently introduces a spurious warning involving inherited
   properties in protocols or categories; however, it also fixes some
   situations where we were failing to emit a warning. I will scrub
   this code tomorrow and fix this issue as well as number of other
   missed warnings / error situations that appear to exist.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55407 91177308-0d34-0410-b5e6-96231b3b80d8