ClangImporter: More principled use of mapType{Into,OutOf}Context()

Before we would construct types containing a mix of interface and
contextual types, and then map them in and out. Straighten this out.

Note that I've also had to start untangling the issue where
synthesized ParamDecls do not have an interface type.

lib/Sema/CodeSynthesis.cpp

@@ -2106,6 +2106,7 @@ swift::createDesignatedInitOverride(TypeChecker &tc,
         // Apply the superclass substitutions to produce an interface
         // type in terms of the class generic signature.
         auto paramSubstTy = paramTy.subst(moduleDecl, subsMap, SubstOptions());
+        decl->setInterfaceType(paramSubstTy);
 
         // Map it to a contextual type in terms of the class's archetypes.
         decl->overwriteType(ArchetypeBuilder::mapTypeIntoContext(