Add forward declaration of operator<< in <string_view> as required.

This declaration was previously missing despite appearing in the
synopsis. Users are still required to include <ostream> to get the
definition of the streaming operator.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@372909 91177308-0d34-0410-b5e6-96231b3b80d8