libstdc++: Fix availability of flat_meow::operator=(initializer_list)

This assignment operator was not being brought in from the private base
class causing assignments from {...} to be inefficiently treated as
construction + move assignment.

libstdc++-v3/ChangeLog:

	* include/std/flat_map (flat_map): Bring in operator= from
	_Flat_map_base.
	(flat_multimap): Likewise.
	* include/std/flat_set (flat_set): Bring in operator= from
	_Flat_set_base.
	(flat_multiset): Likewise.
	* testsuite/23_containers/flat_map/1.cc (test11): Simplify by
	using = {...}.
	(test12): New test.
	* testsuite/23_containers/flat_multimap/1.cc (test10): Simplify
	by using = {...}.
	(test11): New test.
	* testsuite/23_containers/flat_multiset/1.cc (test10): Simplify
	by using = {...}.
	(test11): New test.
	* testsuite/23_containers/flat_set/1.cc (test10): Simplify by
	using = {...}.
	(test11): New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>