Experimental support for a meaningful __is_swappable<T>::value.  This does not appear to be strictly needed for correct functioning of the library.  If it causes any problems, I'd rather pull it sooner rather than later.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@132421 91177308-0d34-0410-b5e6-96231b3b80d8