FnAbi computation should be cached.

[eddyb]

We might be spending a non-trivial amount of time creating FnAbis and adjusting them, so we should investigate caching them, similar to how we cache LLVM Types we compute from Rust Tys.

After #65947, this is what's left:

• FnAbi::of_instance: takes ty::Instance, easy to cache
  • we should be able to make this a query, like layout_of is
• FnAbi::of_fn_ptr: takes ty::PolyFnSig, might be more expensive
  • well, inputs_and_outputs is compared/hashed by pointer nowadays, isn't it?
  • creating LLVM Types for fn pointers is already cached itself, but calls of fn pointers also require FnAbis
  • this is likely used less than FnAbi::of_instance, so it's lower priority

[traxys]

Is it a good issue if I gained some knowledge on the structures of query ? (I would like to learn more of the inner workings of the compiler)

[eddyb]

@traxys Sounds good! You would just need to wait for #65947.