c++: Fix up name independent decl in structured binding handling in range for [PR115586]

cp_parser_range_for temporarily reverts IDENTIFIER_BINDING changes
to hide the decls from the structured bindings from lookup during
parsing of the expression after :
If there are 2 or more name independent decls, we undo IDENTIFIER_BINDING
for the same name multiple times, even when just one has been added
(with a TREE_LIST inside of it as decl).

The following patch fixes it by handling the _ name at most once, the
later loop will DTRT then and just reinstall the temporarily hidden
binding with the TREE_LIST in there.

2025-02-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/115586
	* parser.cc (cp_parser_range_for): For name independent decls in
	structured bindings, only push the name/binding once per
	structured binding.

	* g++.dg/cpp26/name-independent-decl9.C: New test.
	* g++.dg/cpp26/name-independent-decl10.C: New test.