Give SmallPtrSet move semantics when we have R-value references.

Somehow, this ADT got missed which is moderately terrifying considering
the efficiency of move for it.

The code to implement move semantics for it is pretty horrible
currently but was written to reasonably closely match the rest of the
code. Unittests that cover both copying and moving (at a basic level)
added.

llvm-svn: 195239