string_view enhancements. Move to the correct namespace. Better constexpr support (thanks to Richard for the suggestions). Update the tests to match this. Add <experimental/__config for experimental macros/etc to live.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@212569 91177308-0d34-0410-b5e6-96231b3b80d8