Set DECL_ARTIFICIAL on gfortran internal variables.

It seems we sometimes use DECL_ARTIFICIAL as choosing between
different code paths.  In order not to make -fdebug-aux-vars
do different things, set DECL_ARTIFICIAL on the variables to
avoid these different code paths (and the corresponding
regressions).

gcc/fortran/ChangeLog:

	* trans.c (create_var_debug_raw): Set DECL_ARTIFICIAL
	on variables.