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.

From-SVN: r230113