Support unqualified lookup into extended interfaces from classes and impls (#7217)

When a class extends an interface, referring to a member name of the
interface as an unqualified name should refer to the class's
corresponding associated entity value, not to the associated entity
itself. Similarly, in an `impl`, unqualified names of associated
entities should refer to the `impl`'s corresponding value for that
entity.

To support this, we treat `impl`s as `extend`ing their implemented facet
type, and we make lookups into an extended facet type use the `Self`
type of the extending `impl` or `class` if lookup finds an associated
entity. We already did the latter if the extending entity was an
interface; this extends the existing support for these other cases.

toolchain/check/modifiers.cpp

@@ -227,8 +227,8 @@ auto RequireDefaultFinalOnlyInInterfaces(Context& context,
                                          DeclIntroducerState& introducer,
                                          SemIR::NameScopeId parent_scope_id)
     -> void {
-  if (parent_scope_id.has_value() &&
-      context.name_scopes().Get(parent_scope_id).is_interface_definition()) {
+  if (context.name_scopes().InstIs<SemIR::InterfaceWithSelfDecl>(
+          parent_scope_id)) {
     // Both `default` and `final` allowed in an interface definition.
     return;
   }