c++/reflection: splices and passing ctx to finish_id_expr

c++/reflection: splices and passing ctx to finish_id_expr

In <https://gcc.gnu.org/pipermail/gcc-patches/2026-January/705175.html>
(bottom of the message) we discussed not passing ctx to finish_id_expression
so that we can get rid of the _deferring_access_checks calls.

We can avoid passing context to finish_id_expression but we can't
completely avoid the _deferring_access_checks calls, because for
address_p we need to call build_offset_ref which needs it.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_splice_expression): For dependent splices return
	earlier.  Refactor.  For address_p, build an OFFSET_REF.  Don't pass
	context to finish_id_expression.
	* semantics.cc (finish_id_expression_1): Adjust an assert to also check
	flag_reflection.

Reviewed-by: Jason Merrill <jason@redhat.com>