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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170039 91177308-0d34-0410-b5e6-96231b3b80d8