Add a FoldingSetVector datastructure which is analogous to a SetVector,

Add a FoldingSetVector datastructure which is analogous to a SetVector,
but using a FoldingSet underneath and with a largely compatible
interface to that of FoldingSet. This can be used anywhere a FoldingSet
would be natural, but iteration order is significant. The initial
intended use case is in Clang's template specialization lists to
preserve instantiation order iteration.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156131 91177308-0d34-0410-b5e6-96231b3b80d8