Change arena_cache to not alter the declared query result

[Zoxc]

This makes the return types a bit clearer, limiting arena_cache's effect to just the computation side. It also makes it easier to potentially remove arena_cache.

r? @cjgillot

[cjgillot]

This is just syntax changes and type aliases, should not cause any perf effect.
@bors r+ rollup

[bors]

📌 Commit 94f67e0 has been approved by cjgillot

It is now in the queue for this repository.

[matthiaskrgr]

@bors r-
https://github.com/rust-lang/rust/actions/runs/4121278862/jobs/7116811482

    Checking rustc_codegen_ssa v0.0.0 (/checkout/compiler/rustc_codegen_ssa)
    Checking rustc_resolve v0.0.0 (/checkout/compiler/rustc_resolve)
error[E0308]: mismatched types
   --> compiler/rustc_codegen_ssa/src/back/symbol_export.rs:442:48
    |
442 |     providers.upstream_monomorphizations_for = upstream_monomorphizations_for_provider;
    |                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected reference, found enum `Option`
    |
    = note: expected fn pointer `for<'tcx> fn(TyCtxt<'tcx>, DefId) -> &'tcx Option<&'tcx HashMap<&'tcx rustc_middle::ty::List<rustc_middle::ty::GenericArg<'tcx>>, CrateNum, BuildHasherDefault<FxHasher>>>`
                  found fn item `for<'a> fn(TyCtxt<'a>, DefId) -> Option<&'a HashMap<..., ..., ...>> {upstream_monomorphizations_for_provider}`

[Zoxc]

Rebased and fixed the error.

[cjgillot]

@bors r+

[bors]

📌 Commit a0b96fd has been approved by cjgillot

It is now in the queue for this repository.