Auto merge of #101483 - oli-obk:guaranteed_opt, r=fee1-dead

The `<*const T>::guaranteed_*` methods now return an option for the unknown case

cc rust-lang/rust#53020 (comment)

I chose `0` for "not equal" and `1` for "equal" and left `2` for the unknown case so backends can just forward to raw pointer equality and it works ✨

r? `@fee1-dead` or `@lcnr`

cc `@rust-lang/wg-const-eval`

Author: bors