[Ada] Set Current_Error_Node directly

[Ada] Set Current_Error_Node directly

This changes gigi to set Current_Error_Node directly, which should
result in a more robust error handling.

2018-09-26  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* gcc-interface/gigi.h (error_gnat_node): Delete.
	* gcc-interface/trans.c (error_gnat_node): Likewise.
	(gigi): Replace it with Current_Error_Node.
	(gnat_to_gnu): Likewise.
	* gcc-interface/utils.c (rest_of_subprog_body_compilation):
	Likewise.
	* gcc-interface/misc.c (internal_error_function): Do not set it.

From-SVN: r264607