Provide a public API to obtain IngredientIndex

[Y-Nak]

Currently, It seems there is no public API that allows to obtain IngredientIndex from an ingredient itself.
The main motivation for this change is to make it possible to filter out cycle participants by IngredientIndex of a specific tracked function.

The one use case would be like the one below.

// NOTE: `lower_type_alias` and `lower_hir_ty` depend on each other.

#[salsa::tracked(return_ref, recovery_fn = recover_lower_type_alias_cycle)]
pub(crate) fn lower_type_alias(
    db: &dyn Db,
    alias: HirTypeAlias,
) -> Result<TyAlias, AliasCycle> {
    let hir_ty = alias.ty(db);
    let resolved_to = lower_hir_ty(db, hir_ty);

    Ok(TyAlias { resolved_to, ... })
}

#[salsa::tracked]
pub(crate) fn lower_hir_ty(db: &dyn Db, hir_ty: HirTy) -> TyId {
    ...
    // It turns out that `hir_ty` points to a type alias, so we need to lower it first.
    let ty_alias = lower_type_alias(db, alias).resolved_to;
    ...
}

fn recover_lower_type_alias_cycle(
    db: &dyn Db,
    cycle: &salsa::Cycle,
    _alias: HirTypeAlias,
) -> Result<TyAlias, AliasCycle> {
    // Finds ingredient index for `lower_type_alias` function to filter out the
    // cycle participants.
    let (jar, _): (&crate::Jar, _) = db.jar();
    let function =
        &<_ as salsa::storage::HasIngredientsFor<lower_type_alias>>::ingredient(jar).function;
    let ingredient_index = <FunctionIngredient<lower_type_alias> as Ingredient<
        dyn HirAnalysisDb,
    >>::ingredient_index(&function);

    // Here we want to find *ONLY* type aliases that are part of the cycle(i.e.,
    // ignore the `HirTy` in `lower_hir_ty` function).
    let alias_cycle = cycle
        .participant_keys()
        .filter_map(|key| {
            if ingredient_index == key.ingredient_index() {
                Some(lower_type_alias::key_from_id(key.key_index()))
            } else {
                None
            }
        })
        .collect();
    Err(AliasCycle(alias_cycle))
}

If this approach is Ok, I'd be happy to add some wrapper APIs for easy IngredientIndex retrieval.

[nikomatsakis]

Hmm, @Y-Nak -- sorry for the delay -- I don't see an obvious problem with letting people get access to the IngredientIndex. To be honest, I don't have salsa in cache right now, though I am hoping to be able to come back to it next year and start hacking again. I'm going to merge this for now!

bors r+

[nikomatsakis]

Whoops, I forget if we were using bors or what. :)