Function lower_function 

pub fn lower_function<'db>(
    encapsulating_ctx: &mut EncapsulatingLoweringContext<'db>,
    function_id: FunctionWithBodyId<'db>,
    signature: EnrichedSemanticSignature<'db>,
    block_expr_id: ExprId,
) -> Maybe<Lowered<'db>>

Lowers a function into Lowered.