[analyzer] Introduce proper diagnostic for freeing unowned object

Insert a note when the object becomes not (exclusively) owned.

Differential Revision: https://reviews.llvm.org/D56891

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@351514 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed