Add a macro for introducer tokens. (#4031)

Playing with the macro suggestion on #4028, replace DeclKind with some
templating that asserts the in-use token is an introducer token. This
allows type-safe usage of Lex::TokenKind, reducing the benefit of a
separate enum while improving stdout (since now this will ostream as the
keyword name).

toolchain/check/handle_loop_statement.cpp

@@ -77,7 +77,7 @@ auto HandleForHeaderStart(Context& context, Parse::ForHeaderStartId node_id)
 }
 
 auto HandleForIn(Context& context, Parse::ForInId node_id) -> bool {
-  context.decl_introducer_state_stack().Pop(DeclIntroducerState::Var);
+  context.decl_introducer_state_stack().Pop<Lex::TokenKind::Var>();
   return context.TODO(node_id, "HandleForIn");
 }