Implement LWG 2221: formatted output operator for nullptr

Implement LWG 2221: formatted output operator for nullptr

2019-01-10  Ville Voutilainen  <ville.voutilainen@gmail.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	Implement LWG 2221
	* config/abi/pre/gnu.ver (GLIBCXX_3.4): Tighten patterns.
	(GLIBCXX_3.4.26): Add new exports.
	* include/Makefile.am: Add ostream-inst.cc. Move string-inst.cc to
	correct list of sources.
	* include/Makefile.in: Regenerate.
	* include/std/ostream (operator<<(nullptr_t)): New member function.
	* src/c++17/ostream-inst.cc: New file.
	* testsuite/27_io/basic_ostream/inserters_other/char/lwg2221.cc: New
	test.

Co-Authored-By: Jonathan Wakely <jwakely@redhat.com>
From-SVN: r267808