Fix missing back-annotation for derived types

Fix missing back-annotation for derived types

Gigi fails to back-annotate the Present_Expr field of variants present
in a type derived from a discriminated untagged record type, which is
for example visible in the output -gnatRj.

gcc/ada/ChangeLog
	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Record_Type>: Tidy up.
	(build_variant_list): Add GNAT_VARIANT_PART parameter and annotate
	its variants if it is present.  Adjust the recursive call by passing
	the variant subpart of variants, if any.
	(copy_and_substitute_in_layout): Rename GNU_SUBST_LIST to SUBST_LIST
	and adjust throughout.  For a type, pass the variant part in the
	call to build_variant_list.