libstdc++: Inconsistent const in flat_map's value_type [PR122921]

libstdc++: Inconsistent const in flat_map's value_type [PR122921]

flat_map's value_type is pair<key_type, mapped_type>, which we correctly
define within the container but incorrectly within the iterator.

	PR libstdc++/122921

libstdc++-v3/ChangeLog:

	* include/std/flat_map (_Flat_map_impl::_Iterator::value_type):
	Remove const from key_type to make consistent with the
	container's value_type.
	* testsuite/23_containers/flat_map/1.cc (test09): New test.
	* testsuite/23_containers/flat_multimap/1.cc (test09): New test.

Reported-by: Vincent X
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>