StringMap support for move-only values.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208359 91177308-0d34-0410-b5e6-96231b3b80d8
StringMap: Replace faux-copyability with faux-movability, which is sufficient.

This behavior was added to support StringMaps of StringMaps, default +
move construction are sufficient for this.

Real move construction support coming soon (& probably copy construction
too).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208360 91177308-0d34-0410-b5e6-96231b3b80d8
StringMap: Move assignment and move construction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208361 91177308-0d34-0410-b5e6-96231b3b80d8