Draft: Ensure inferred patterns have a fixed RuntimeRep

This patch adds a field ir_frr to the InferResult datatype, holding a value of type Maybe FRROrigin. When this value is not Nothing, this means that we must fill the ir_ref field with a type which has a fixed RuntimeRep. When it comes time to fill such an ExpType, we ensure that the type has a fixed RuntimeRep by performing a representation-polymorphism check with the given FRROrigin This is similar to what we already do to ensure we fill an Infer ExpType with a type of the correct TcLevel.

I had to move fillInferResult from GHC.Tc.Utils.TcMType to GHC.Tc.Utils.Unify to avoid creating cyclic imports.

This patch also changes the code in matchExpectedFunTysto make use of the coercions, which is now possible thanks to the previous change. This implements PHASE 2 of FixedRuntimeRep in some situations. For example, the test cases T13105 and T17536b are now both accepted.

I've also taken the opportunity to change the type returned by hasFixedRuntimeRep, so that it directly answers with the most useful coercion (which is a GRefl), as opposed to the kind coercion, and to use MCoercion instead of Maybe as suggested by @rae.