gccrs: derive: Add dummy builtin transcriber for builtin derive macros.

gccrs: derive: Add dummy builtin transcriber for builtin derive macros.

Fixes #2257.

Derive builtins do not need a real transcriber, but still need one. It
will however never be called since builtin derive macros get expanded
differently, and benefit from knowing on what kind of items they are
applied (struct, enums, unions) rather than receiving a list of tokens
like regular builtin macros.

gcc/rust/ChangeLog:

	* expand/rust-macro-builtins.cc (MacroBuiltin::dummy): New function.
	* expand/rust-macro-builtins.h: Declare it.

gcc/testsuite/ChangeLog:

	* rust/compile/derive_macro7.rs: New test.