[gfortran] Assorted matching fixes, fix No 1 from PR16404

[Tobias Schlüter]

Tobias Schlüter wrote:
> +       gfc_error_now("Variable '%s' at %C is a DO-iterator and cannot be "
> +                     "redefined", st->name);

I changed this to
	gfc_error_now("Variable '%s' at %C cannot be redefined inside "
		      "loop beginning at %L", st->name, &s->tail->loc);
which seems friendlier.

- Tobi