LoongArch: Fix C23 (...) functions returning large aggregates [PR114175]

LoongArch: Fix C23 (...) functions returning large aggregates [PR114175]

We were assuming TYPE_NO_NAMED_ARGS_STDARG_P don't have any named
arguments and there is nothing to advance, but that is not the case
for (...) functions returning by hidden reference which have one such
artificial argument.  This is causing gcc.dg/c23-stdarg-6.c and
gcc.dg/c23-stdarg-8.c to fail.

Fix the issue by checking if arg.type is NULL, as r14-9503 explains.

gcc/ChangeLog:

	PR target/114175
	* config/loongarch/loongarch.cc
	(loongarch_setup_incoming_varargs): Only skip
	loongarch_function_arg_advance for TYPE_NO_NAMED_ARGS_STDARG_P
	functions if arg.type is NULL.

(cherry picked from commit c1fd4589c2bf9fd8409d51b94df219cb75107762)