libstdc++: allow string_view insertion to work with <iosfwd> (PR 94051)

I don't think this is actually required to compile, because using
operator<< without a definition of the ostream doesn't seem valid to me.
But it's easy to make it work.

	PR libstdc++/94051
	* include/std/string_view: Include <bits/ostream_insert.h>.
	* testsuite/21_strings/basic_string_view/inserters/94051.cc: New test.

libstdc++-v3/ChangeLog

+2020-03-05  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/94051
+	* include/std/string_view: Include <bits/ostream_insert.h>.
+	* testsuite/21_strings/basic_string_view/inserters/94051.cc: New test.
+
 2020-03-04  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/cpp_type_traits.h (__memcpyable): Fix comment.

libstdc++-v3/include/std/string_view

@@ -42,6 +42,7 @@
 #include <bits/functional_hash.h>
 #include <bits/int_limits.h>
 #include <bits/range_access.h>
+#include <bits/ostream_insert.h>
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {

libstdc++-v3/testsuite/21_strings/basic_string_view/inserters/94051.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++17" }
+// { dg-do compile { target c++17 } }
+
+#include <iosfwd>
+#include <string_view>
+
+void
+test01(std::ostream& os, std::string_view sv)
+{
+  os << sv;
+}