Note [FixedRuntimeRep context in ExpType]

Sometimes, we want to be sure that we fill an ExpType with a type
that has a syntactically fixed RuntimeRep (in the sense of
Note [Fixed RuntimeRep] in GHC.Tc.Utils.Concrete).

Example:

  pattern S a = (a :: (T :: TYPE R))

We have to infer a type for `a` which has a syntactically fixed RuntimeRep.
When it comes time to filling in the inferred type, we do the appropriate
representation-polymorphism check, much like we do a level check
as explained in Note [TcLevel of ExpType].

See test case T21325.