Couple of tweaks to help in LTO mode

Couple of tweaks to help in LTO mode

The first tweak is to remove the TREE_OVERFLOW flag on INTEGER_CSTs
because it prevents them from being uniquized in LTO mode.

The second, unrelated tweak is to canonicalize the packable types made
by gigi so that at most one per type is present in the GENERIC IL.

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Array_Subtype>: Deal
	with artificial maximally-sized types designed by access types.
	* gcc-interface/utils.c (packable_type_hash): New structure.
	(packable_type_hasher): Likewise.
	(packable_type_hash_table): New hash table.
	(init_gnat_utils): Initialize it.
	(destroy_gnat_utils): Destroy it.
	(packable_type_hasher::equal): New method.
	(hash_packable_type): New static function.
	(canonicalize_packable_type): Likewise.
	(make_packable_type): Make sure not to use too small a type for the
	size of the new fields.  Canonicalize the type if it is named.