[temp.constr.order] Subsumption is a preorder, not a partial ordering #9371

The note in [temp.constr.order] p2 says:

The subsumption relation defines a partial ordering on constraints.

It does not. By the definition in p1, A && B and B && A subsume each other and are different constraints, so the relation is not antisymmetric. It is a preorder: reflexive and transitive, but with distinct constraints that subsume each other.

The partial ordering the note goes on to list uses for is the ordering of declarations that p4 and p5 ("at least as constrained", "more constrained") induce from subsumption, which is a strict partial order.

Nothing normative depends on the sentence, since every normative use of subsumption asks for one direction and whether the converse fails. The note should say what the relation is and which ordering is used.

Proposed fix in #9368.