how to check the global cache

[lcnr]

I don't know where else to put it, so:

We can test that the global cache is correct by randomly ignoring it when proving goal and checking that its result is equal to the recomputed one. We must not use the global cache if the current goal is already in the provisional result.

I did this as part of rust-lang/rust#114287 and it worked when compiling typenum and our UI tests, we should try this again in a crater run before stabilization however.

[lcnr]

now just supported as part of the fuzzing impl