Refine ObjCPropertyImplDecl:

Refine ObjCPropertyImplDecl:

- Remove a bunch of setXXX methods until we actually need them. This cleans up the
interface, and makes the object immutable until we have a reason to be mutable.

- Remove enum value OBJC_PR_IMPL_None since it was never used.

- Remove instance variable 'PropertyImplKind PropertyImplementation;'. This is
  not needed because we can tell if ObjCPropertyImplDecl represents @synthesize
  if 'ObjCPropertyDecl *PropertyDecl' is not null.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@54055 91177308-0d34-0410-b5e6-96231b3b80d8