Make the moved-from SmallPtrSet be a valid, empty, small-state object.

Make the moved-from SmallPtrSet be a valid, empty, small-state object.
Enhance the tests to actually require moves in C++11 mode, in addition
to testing the moved-from state. Further enhance the tests to cover
copy-assignment into a moved-from object and moving a large-state
object. (Note that we can't really test small-state vs. large-state as
that isn't an observable property of the API really.) This should finish
addressing review on r195239.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195261 91177308-0d34-0410-b5e6-96231b3b80d8