libstdc++: Add unordered containers heterogeneous lookup

libstdc++: Add unordered containers heterogeneous lookup

Add unordered containers heterogeneous lookup member functions find, count, contains and
equal_range in C++20. Those members are considered for overload resolution only if hash and
equal functors used to instantiate the container have a nested is_transparent type.

libstdc++-v3/ChangeLog:

	* include/bits/stl_tree.h
	(__has_is_transparent, __has_is_transparent_t): Move...
	* include/bits/stl_function.h: ...here.
	* include/bits/hashtable_policy.h (_Hash_code_base<>::_M_hash_code_tr): New..
	(_Hashtable_base<>::_M_equals_tr): New.
	* include/bits/hashtable.h (_Hashtable<>::_M_find_tr, _Hashtable<>::_M_count_tr,
	_Hashtable<>::_M_equal_range_tr): New member function templates to perform
	heterogeneous lookup.
	(_Hashtable<>::_M_find_before_node_tr): New.
	(_Hashtable<>::_M_find_node_tr): New.
	* include/bits/unordered_map.h (unordered_map::find<>, unordered_map::count<>,
	unordered_map::contains<>, unordered_map::equal_range<>): New member function
	templates to perform heterogeneous lookup.
	(unordered_multimap::find<>, unordered_multimap::count<>,
	unordered_multimap::contains<>, unordered_multimap::equal_range<>): Likewise.
	* include/bits/unordered_set.h  (unordered_set::find<>, unordered_set::count<>,
	unordered_set::contains<>, unordered_set::equal_range<>): Likewise.
	(unordered_multiset::find<>, unordered_multiset::count<>,
	unordered_multiset::contains<>, unordered_multiset::equal_range<>): Likewise.
	* include/debug/unordered_map
	(unordered_map::find<>, unordered_map::equal_range<>): Likewise.
	(unordered_multimap::find<>, unordered_multimap::equal_range<>): Likewise.
	* include/debug/unordered_set
	(unordered_set::find<>, unordered_set::equal_range<>): Likewise.
	(unordered_multiset::find<>, unordered_multiset::equal_range<>): Likewise.
	* testsuite/23_containers/unordered_map/operations/1.cc: New test.
	* testsuite/23_containers/unordered_multimap/operations/1.cc: New test.
	* testsuite/23_containers/unordered_multiset/operations/1.cc: New test.
	* testsuite/23_containers/unordered_set/operations/1.cc: New test.