Only apply adjust_args in OpenMP dispatch if variant substitution occurs

This is a followup to
084ea8ad OpenMP: middle-end support for dispatch + adjust_args.

This patch fixes a bug that caused arguments in an OpenMP dispatch call to be
modified even when no variant substitution occurred.

gcc/ChangeLog:

	* gimplify.cc (gimplify_call_expr): Create variable
	variant_substituted_p to control whether adjust_args applies.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/adjust-args-4.c: New test.