The function TcHsType.failIfEmitsConstraints says that it fails.
It even does so in its name. But it didn't! It *reported* constraints
but didn't fail. Now it does.

This is important in tcHsClsInstType; see the comments therein.

This was discovered while looking at #15797, but that ticket
requires visible kind application to exhibit the bug; the test
case will come with the patch for #12045.