* include/bits/iterator_concepts.h (__iter_concept_impl): Add
	comments.
	(indirect_relation): Rename to indirect_binary_predicate and adjust
	definition as per P1716R3.
	(indirect_equivalence_relation): Define.
	(indirectly_comparable): Adjust definition.
	* include/std/concepts (equivalence_relation): Define.
	* testsuite/std/concepts/concepts.callable/relation.cc: Add tests for
	equivalence_relation.

From-SVN: r278256