libstdc++: Support maps deduction from_range of tuples.

libstdc++: Support maps deduction from_range of tuples.

This implements part of LWG4223 that enables deduction for maps types
(map, unordered_map, flat_map and non-unique equivalent) from
(from_range, rg, ...) arguments, where rg is range of tuple
or other pair-like.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (__detail::__range_key_type):
	Replace RV::first_type with tuple_element_t<0, RV>.
	(__detail::__range_mapped_type) Replace RV::second_type
	with tuple_element_t<1, RV>.
	* testsuite/23_containers/flat_map/1.cc: New tests.
	* testsuite/23_containers/flat_multimap/1.cc: New tests.
	* testsuite/23_containers/map/cons/from_range.cc: New tests.
	* testsuite/23_containers/multimap/cons/from_range.cc: New tests.
	* testsuite/23_containers/unordered_map/cons/from_range.cc: New tests.
	* testsuite/23_containers/unordered_multimap/cons/from_range.cc:
	New tests.

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