Fix PR28079 - std::wstring_convert move constructor broken.

The move constructor for wstring_convert accidentally copied the state member
into the converted count member in the move constructor. This patch fixes
the typo.

While working on this I discovered that wstring_convert doesn't actually
provide a move constructor according to the standard and therefore this
constructor is a libc++ extension. I'll look further into whether libc++ should
provide this constructor at all. Neither libstdc++ or MSVC's STL provide it.

llvm-svn: 273831