Remove WithCachedTypeInfo::stable_hash.

We store a stable hash value in the most common interned values (e.g.
types, predicates, regions). This is 16 bytes of data.

- In non-incremental builds it's a straightforward performance loss: the
  stable hash isn't computed or used, and the 16 bytes of space goes to
  waste (but it still gets hashed when interning).

- In incremental builds it avoids some hashing but doesn't seem to
  actually be a genuine performance win, and the complexity doesn't seem
  worth it.