libstdc++: Fix tuple/pair confusion with std::erase_if(flat_map) [PR120465]

libstdc++: Fix tuple/pair confusion with std::erase_if(flat_map) [PR120465]

std::erase_if for flat_map/multimap is implemented via ranges::erase_if
over a zip_view of the keys and values, the value_type of which is a
tuple, but the given predicate needs to be called with a pair (flat_map's
value_type).  So use a projection to convert the tuple into a suitable
pair.

	PR libstdc++/120465

libstdc++-v3/ChangeLog:

	* include/std/flat_map (_Flat_map_impl::_M_erase_if): Use a
	projection with ranges::remove_if to pass a pair instead of
	a tuple to the predicate.
	* testsuite/23_containers/flat_map/1.cc (test07): Strengthen
	to expect the argument passed to the predicate is a pair.
	* testsuite/23_containers/flat_multimap/1.cc (test07): Likewise.

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