Speeding up type lookup

[Martin v. Loewis]

> The right thing to do is to use the new automatically expanding hash
> tables in libiberty, and ditch the ad-hoc hash table in tree.c.  That
> will solve the problem, even for very large compilations.  Would you
> be willing to tackle that instead?

Sure. Do we need another version of ggc_add_tree_hash_table_root (one
that operates on libiberty hashtab.c), or should I just register the
type hash as a root with a custom callback?

Martin