Implement n3545 for c++14

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185856 91177308-0d34-0410-b5e6-96231b3b80d8
Implement n3668 - std::exchange

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185863 91177308-0d34-0410-b5e6-96231b3b80d8