Allow simple uses of Self in impls. (#3824)

Per #3714, some of the details here are not yet settled. In particular,
we might want `Self` to come into scope at the start of the definition,
not at the `as` keyword. However, this change allows us to accept the
uncontroversial examples.

toolchain/check/handle_interface.cpp

@@ -81,7 +81,6 @@ static auto BuildInterfaceDecl(Context& context,
     // there was an error in the qualifier, we will have lost track of the
     // interface name here. We should keep track of it even if the name is
     // invalid.
-    // TODO: should have a `Self` type id member
     interface_decl.interface_id = context.interfaces().Add(
         {.name_id = name_context.name_id_for_new_inst(),
          .enclosing_scope_id = name_context.enclosing_scope_id_for_new_inst(),