Improve pretty-printer defaulting logic (#19361)
As described in Note [Defaulting RuntimeRep variables], GHC should perform defaulting in signatures forall (r :: RuntimeRep). ...
but not when printing an error message r ~ LiftedRep
.
Previously, this distinction was made by remembering whether we are printing a kind. However, this test could not be used with linear types. Unlike RuntimeRep, Multiplicity is used in types rather than kinds. This caused #19361.
Here I am changing the condition: perform defaulting for metavariables but not skolems.
The output in T15883* changes, but I think the defaulting was overeager previously.