Objective-C @synthesize support.

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@55365 91177308-0d34-0410-b5e6-96231b3b80d8