Function lower_loop_function 

pub fn lower_loop_function(
    encapsulating_ctx: &mut EncapsulatingLoweringContext<'_>,
    function_id: FunctionWithBodyId,
    loop_signature: Signature,
    loop_ctx: LoopContext,
    return_type: TypeId,
) -> Maybe<Lowered>

Lowers a loop inner function into Lowered. Similar to lower_function, but adds a recursive call.