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/handle_interface.cpp

@@ -183,7 +183,7 @@ auto HandleParseNode(Context& context,
   // Set on the name scope that `M` is replaced by `Self.M`.
   context.name_scopes()
       .Get(interface_info.scope_with_self_id)
-      .set_is_interface_definition();
+      .set_self_type_id(interface_info.self_param_id);
 
   // Start the definition of interface-with-self.
   StartGenericDefinition(context, interface_info.generic_with_self_id);