[objc] For the ARC error that is emitted when a synthesized property implementation

has inconsistent ownership with the backing ivar, point the error location to the
ivar.

Pointing to the ivar (instead of the @synthesize) is better since this is where a fix is needed.
Also provide the location of @synthesize via a note.

This also fixes the problem where an auto-synthesized property would emit an error without
any location.

llvm-svn: 170039