Completing a type no longer ignores facet types (#5004)

Make facet types complete like other types. This means that in the body
of an interface, the type of `Self` is incomplete. This involved fixing
an issue where eval of a specific_id that was already canonical was not
resolving the specific declaration, which could occur as part of
substituting into a facet type.

---------

Co-authored-by: Josh L <josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/handle_impl.cpp

@@ -322,7 +322,13 @@ static auto CheckConstraintIsInterface(Context& context,
 
   auto complete_id = RequireCompleteFacetType(
       context, facet_type_id, context.insts().GetLocId(impl.constraint_id),
-      *facet_type, FacetTypeImpl);
+      *facet_type, [&] {
+        CARBON_DIAGNOSTIC(ImplAsIncompleteFacetType, Error,
+                          "impl as incomplete facet type {0}", InstIdAsType);
+        return context.emitter().Build(impl.latest_decl_id(),
+                                       ImplAsIncompleteFacetType,
+                                       impl.constraint_id);
+      });
   if (!complete_id.has_value()) {
     return nullptr;
   }