swift234

Replace use of getArchetype() in diagnoseGenericParameterErrors()

This is a quick follow-up to
<https://github.com/apple/swift/pull/1160>, to replace the becoming
deprecated getArchetype() with doing the same thing via calling through
to ArchetypeBuilder with a declContext.

Uses findGenericSubstitutions() to do so. So this site could take
advantage of destructuring of more complex params containing generics.
Right now, though, that never happens. For complex params the
(unfortunately, worse) diagnosis happens in diagnoseFailureForExpr() on
the argument expression before reaching here. I’d like to improve that
in future work.