openmp: Teach OpenMP declare variant append_args handling about TYPE_NO_NAMED_ARGS_ST...

openmp: Teach OpenMP declare variant append_args handling about TYPE_NO_NAMED_ARGS_STDARG_P

Since my recent patch, GCC for C++26 uses the TYPE_NO_NAMED_ARGS_STDARG_P
flag like C23 uses for (...) function types.  The OpenMP declare variant
append_args handling does some very ugly hacks (modify TYPE_ARG_TYPES
temporarily instead of trying to create new function types) and had
to be tweaked to deal with that.  This fixes
-FAIL: c-c++-common/gomp/append-args-7.c  -std=c++26  scan-tree-dump-times gimple "f3 \\\\(obj1, obj2, 1, a, cp, d\\\\);" 1
-FAIL: c-c++-common/gomp/append-args-7.c  -std=c++26 (test for excess errors)

2025-10-13  Jakub Jelinek  <jakub@redhat.com>

	* decl.cc (omp_declare_variant_finalize_one): If !nbase_args
	and TREE_TYPE (decl) has TYPE_NO_NAMED_ARGS_STDARG_P bit set
	and varg is NULL, temporarily set TYPE_NO_NAMED_ARGS_STDARG_P
	on TREE_TYPE (variant).