libstdc++-v3: Implement allocator-aware from_range_t constructors for unordered conta...

libstdc++-v3: Implement allocator-aware from_range_t constructors for unordered containers.

This patch implements part of LWG2713 covering the from_range
constructors, which makes std::ranges::to<std::unordered_set>(alloc)
well-formed. Likewise for rest of unordered containers.

As this consturctors were added to v15, this has no impact
on code that compiled with previous versions.

libstdc++-v3/ChangeLog:

	* include/bits/unordered_map.h
	(unordered_map(from_range_t, _Rg&&, const allocator_type&))
	(unordered_multimap(from_range_t, _Rg&&, const allocator_type&)):
	Define.
	* include/bits/unordered_set.h
	(unordered_set(from_range_t, _Rg&&, const allocator_type&))
	(unordered_multiset(from_range_t, _Rg&&, const allocator_type&)):
	Define.
	* testsuite/23_containers/unordered_map/cons/from_range.cc: New tests.
	New tests.
	* testsuite/23_containers/unordered_multimap/cons/from_range.cc:
	New tests.
	* testsuite/23_containers/unordered_multiset/cons/from_range.cc:
	New tests.
	* testsuite/23_containers/unordered_set/cons/from_range.cc: New tests.
	* testsuite/std/ranges/conv/1.cc: New tests.

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