Deduplication: Extract a shared helper function to remove duplicate associated const binding lowering

[LaneAsade]

This PR aims to deduplicate the code the responsible for lowering the RHS of associated const bindings (found at bounds.rs and mod.rs) by creating a helper function lower_assoc_const_binding_rhs.

Related: #150621

Helper function code:

pub(crate) fn lower_assoc_const_binding_rhs(
        &self,
        ct: &'tcx hir::ConstArg<'tcx>,
        assoc_ident: Ident,
        constraint_hir_id: hir::HirId,
        projection_term: ty::Binder<'tcx, ty::AliasTerm<'tcx>>,
    ) -> ty::Const<'tcx> {
        let tcx = self.tcx();
        let ty = projection_term
            .map_bound(|alias| tcx.type_of(alias.def_id).instantiate(tcx, alias.args));
        let ty = check_assoc_const_binding_type(self, assoc_ident, ty, constraint_hir_id);
        self.lower_const_arg(ct, ty)
}

[rustbot]

HIR ty lowering was modified

cc @fmease

[rustbot]

r? @jackh726

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[afurm]

Nit: check_assoc_const_binding_type is called as bounds::check_assoc_const_binding_type at the mod.rs call site but as just check_assoc_const_binding_type (direct) at the bounds.rs call site — the helper normalises this to always call it as a method on self, which is correct, but worth verifying the two sites were always semantically equivalent (the FIXME comment in the old mod.rs code hints this was previously uncertain).

[LaneAsade]

As far as I know, the FIXME in the old mod.rs was about code duplication in lowering the RHS type and not about the equivalence of the two call sites. Even then, the two call sites are semantically equivalent so no worries there. Thank you for the feedback though!