libstdc++: make trees hetero equal_range O(lg n) [PR118851]

As shipped, the heterogeneous-key members equal_range of rbtree
containers map, multimap, set, and multiset walk the entire range
calling the predicate, in violation of the requirement for
logarithmic complexity. This patch revises them to match the
behavior of the non-heterogeneous multimap and multiset members,
and provides tests to verify it.

libstdc++-v3/Changelog:
	PR libstdc++/118851
	* include/bits/stl_tree.h (_M_equal_range_tr): Rewrite to match
	non-heterogeneous equal_range implementation.
	* testsuite/23_containers/map/operations/hetero/equal_range.cc:
	New test.
	* testsuite/23_containers/multimap/operations/hetero/equal_range.cc:
	Same.
	* testsuite/23_containers/multiset/operations/hetero/equal_range.cc:
	Same.
	* testsuite/23_containers/set/operations/hetero/equal_range.cc: Same.