swift234

ClangImporter: Fix diagnostics for NS* prefix stripping from generic types

We were failing to create an unavailable TypeAlias for the old name
in the case the renamed type was generic, leading to poor diagnostics.

Also, Sema resolves generic TypeAliases very early, while building
a Type from a TypeRepr -- this means the unavailable/deprecated
check runs too late to catch generic TypeAlises.

Add a hack where we preserve a reference to the original TypeAliasDecl
by way of constructing a SubstitutedType which desugars to the
replacement type, rather than resolving the replacement type
directly, so that the availability check can pick it up.

A better fix for this would be to introduce a BoundGenericAliasType
sugared type, but that's a bigger change that can come later.

Fixes <rdar://problem/26206263>.