Add a test for assignment operator behavior which was changed in
r195239, as well as a comment about the fact that assigning over
a moved-from object was in fact tested. Addresses some of the review
feedback on r195239.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195260 91177308-0d34-0410-b5e6-96231b3b80d8