swift234

Sema: Hacky fix for a crash in CSDiag

It looks like the mapTypeIntoContext() call is using the wrong
'dc', because in fact we have no way of knowing what the original
'dc' was which produced the type parameters in question.

Really the problem is we're picking apart SubstitutedType here,
which is almost always the wrong thing to do. I don't think
SubstitutedType should exist at all.

To avoid the crash, just bail-out if 'dc' is not generic.

I'm intentionally avoiding a principled fix here to hasten the
apocalypse when all of CSDiag.cpp will collapse under its own
weight and get rewritten in a sane way.