PR78134 fix return types of heterogeneous lookup functions

PR78134 fix return types of heterogeneous lookup functions

	PR libstdc++/78134
	* include/bits/stl_map.h (map::lower_bound, map::upper_bound)
	(map::equal_range): Fix return type of heterogeneous overloads.
	* include/bits/stl_multimap.h (multimap::lower_bound)
	(multimap::upper_bound, multimap::equal_range): Likewise.
	* include/bits/stl_multiset.h (multiset::lower_bound)
	(multiset::upper_bound, multiset::equal_range): Likewise.
	* include/bits/stl_set.h (set::lower_bound, set::upper_bound)
	(set::equal_range): Likewise.
	* testsuite/23_containers/map/operations/2.cc
	* testsuite/23_containers/multimap/operations/2.cc
	* testsuite/23_containers/multiset/operations/2.cc
	* testsuite/23_containers/set/operations/2.cc

From-SVN: r244318