[analyzer] Fix non-determinizm introduced in r172104.

In some cases, we just pick any ivar that needs invalidation and attach
the warning to it. Picking the first from DenseMap of pointer keys was
triggering non-deterministic output.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172134 91177308-0d34-0410-b5e6-96231b3b80d8