Make ArchetypeType::NestedType its own proper type

with more explicit/semantic conversions in and out.

Using a PointerUnion with overlapping pointer types
is both error-prone and pretty close to illegible.

Swift SVN r24707

lib/Sema/ConstraintSystem.cpp

@@ -454,8 +454,7 @@ namespace {
                                 
         if (implArchetype->hasNestedType(member->getName())) {
           nestedType = implArchetype->getNestedType(member->getName());
-          archetype = ArchetypeType::getNestedTypeValue(nestedType)
-                        ->getAs<ArchetypeType>();
+          archetype = nestedType.getValue()->getAs<ArchetypeType>();
         } else if (implArchetype->isSelfDerived()) {
           archetype = implArchetype;
         }