Consolidate swap, swap_ranges, and iter_swap in <type_traits>.

NFC. Thanks to @Quuxplusone (Arthur O'Dwyer) for this change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@371639 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed