* gcc-interface/decl.c (subst_pair): Declare. Declare a VEC of it.
(build_subst_list): Return a VEC instead of a tree.
(build_variant_list): Take a VEC for subst_list. Adjust
accordingly.
(create_field_decl_from): Likewise.
(create_variant_part_from): Likewise.
(copy_and_substitute_in_size): Likewise.
(gnat_to_gnu_entity): Adjust for new interface to build_subst_list.
Free the built vector.