Auto merge of #155329 - nnethercote:rm-WithCachedTypeInfo-stable_hash, r=oli-obk

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.

r? @oli-obk

Author: bors