Allow defaulting of representational equalities

This commit generalises the defaulting of equality constraints that
was introduced in 663daf8d (with follow-up in 6863503c) to allow
the defaulting of *representational* equality constraints.

Now we default a representational equality

  ty1 ~R# ty2

by unifying ty1 ~# ty2.

This allows the following defaulting to take place:

  - Coercible alpha[tau] Int ==> alpha := Int
  - Coercible (IO beta[tau]) (IO Char) ==> beta := Char

See Note [Defaulting representational equalities] in GHC.Tc.Solver.Default
for more details.

Fixes #21003