swift234

Rework the interface to coerceParameterListToType to make it obvious
that it is specific to ClosureExprs.  Also, consolidate some logic
in CSDiags into the now shared coerceParameterListToType, which
makes a bit more sense and simplifies things a lot.  NFC.

There are still unanswered questions.  It isn't clear to me why
we support API names on closures, when we don't implement proper
semantic analysis for them. This seems like an accidentally supported
feature that should be removed.