Add tests to make sure that <string_view> provides std::size/data/empty in C++17 mode. This is LWG#3009, coming up for a vote in JAX - but we already do it, just don't have tests

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@323719 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed