Fix return type of heterogeneous find for sets

Fix return type of heterogeneous find for sets

	PR libstdc++/68190
	* include/bits/stl_multiset.h (multiset::find): Fix return types.
	* include/bits/stl_set.h (set::find): Likewise.
	* testsuite/23_containers/map/operations/2.cc: Test find return types.
	* testsuite/23_containers/multimap/operations/2.cc: Likewise.
	* testsuite/23_containers/multiset/operations/2.cc: Likewise.
	* testsuite/23_containers/set/operations/2.cc: Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@230113 138bc75d-0d04-0410-961f-82ee72b054a4