WIP: Broaden the circumstances under which inaccessible code warnings are reported

This came out of the conversation in #17543, where it was desired that we have more warnings before making the ambiguity checker make fewer errors. This change makes GHC a bit more aggressive about providing warnings in cases where constraints are insoluble.

See Trac #12466 (closed) for a long discussion in the context when the warning under discussion was still an error, and its occurrence was restricted to cases involving certain kinds of GADT matches. In Trac #11066 (closed) it became a warning only. In discussing issue #17543, it was found that we would like to see the warning in a broader range of cases again.