Objective-C @synthesize support.

 - Only supports simple assignment and atomic semantics are ignored.
 - Not quite usable yet because the methods do not actually get added
   to the class metadata.
 - Added ObjCPropertyDecl::getSetterKind (one of Assign, Copy, Retain).
 - Rearrange CodeGenFunction so synthesis can reuse function prolog /
   epilog code.

llvm-svn: 55365