libstdc++: Fix tests that use invalid types in ordered containers

libstdc++: Fix tests that use invalid types in ordered containers

Types used in ordered containers need to be comparable, or the container
needs to use a custom comparison function. These tests fail when
_GLIBCXX_CONCEPT_CHECKS is defined, because the element types aren't
comparable.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* testsuite/20_util/is_nothrow_swappable/value.h: Use custom
	comparison function for priority_queue of type with no
	relational operators.
	* testsuite/20_util/is_swappable/value.h: Likewise.
	* testsuite/24_iterators/output/concept.cc: Add operator< to
	type used in set.