c++, libstdc++: Implement P4156R0 - Rename meta::has_ellipsis_parameter to meta::is_vararg_function

Unhappy about the old and new names, as a function type is not a function,
but ces't la vie.

2026-03-30  Jakub Jelinek  <jakub@redhat.com>

gcc/cp/
	* metafns.gperf (enum metafn_code): Remove
	METAFN_HAS_ELLIPSIS_PARAMETER, add METAFN_IS_VARARG_FUNCTION.
	(has_ellipsis_parameter): Remove.
	(is_vararg_function): Add.
	* reflect.cc (eval_has_ellipsis_parameter): Rename to ...
	(eval_is_vararg_function): ... this.  Adjust function comment.
	(process_metafunction): Handle METAFN_IS_VARARG_FUNCTION
	instead of METAFN_HAS_ELLIPSIS_PARAMETER.
	* metafns.h: Regenerate.
gcc/testsuite/
	* g++.dg/reflect/has_ellipsis_parameter1.C: Rename to ...
	* g++.dg/reflect/is_vararg_function1.C: this.  New test.  Rename
	has_ellipsis_parameter to is_vararg_function everywhere.
libstdc++-v3/
	* include/std/meta (has_ellipsis_parameter): Rename to ...
	(is_vararg_function): ... this.  New declaration.
	* src/c++23/std.cc.in: Remove std::meta::has_ellipsis_parameter
	export, add std::meta::is_vararg_function export.