Remove <experimental/string_view>; use <string_view> instead. See...

Remove <experimental/string_view>; use <string_view> instead. See https://libcxx.llvm.org/TS_deprecation.html

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324290 91177308-0d34-0410-b5e6-96231b3b80d8