libstdc++: Use ADL swap for containers' function objects [PR117921]

libstdc++: Use ADL swap for containers' function objects [PR117921]

The standard says that Compare, Pred and Hash objects should be swapped
as described in [swappable.requirements] which means calling swap
unqualified with std::swap visible to name lookup.

libstdc++-v3/ChangeLog:

	PR libstdc++/117921
	* include/bits/hashtable_policy.h (_Hash_code_base::_M_swap):
	Use ADL swap for Hash members.
	(_Hashtable_base::_M_swap): Use ADL swap for _Equal members.
	* include/bits/stl_tree.h (_Rb_tree::swap): Use ADL swap for
	_Compare members.
	* testsuite/23_containers/set/modifiers/swap/adl.cc: New test.
	* testsuite/23_containers/unordered_set/modifiers/swap-2.cc: New
	test.