[Ada] AI12-0028-1 Import of variadic C functions

[Ada] AI12-0028-1 Import of variadic C functions

2020-06-19  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* gcc-interface/decl.c (gnat_to_gnu_param): Tidy up.
	(gnat_to_gnu_subprog_type): For a variadic C function, do not
	build unnamed parameters and do not add final void node.
	* gcc-interface/misc.c: Include snames.h.
	* gcc-interface/trans.c (Attribute_to_gnu): Tidy up.
	(Call_to_gnu): Implement support for unnamed parameters in a
	variadic C function.
	* gcc-interface/utils.c: Include snames.h.
	(copy_type): Tidy up.