Allow defaulting of representational equalities

Issues: #21003 (closed).

This commit generalises the defaulting of equality constraints that was introduced in 663daf8d to allow the defaulting of representational equality constraints.

In practice, this means that if one ends up with an ambiguous type variable a0 but with an unsolved constraint such as Coercible a0 Int, one will simply default a0 := Int instead of reporting a type error. This should allow strictly more programs to be accepted than before.