Insert parentheses around binary operation with attribute

[dtolnay]

Fixes the bug found by @fmease in #134661 (review).

Previously, -Zunpretty=expanded would expand this program as follows:

#![feature(stmt_expr_attributes)]
#![allow(unused_attributes)]

macro_rules! group {
    ($e:expr) => {
        $e
    };
}

macro_rules! extra {
    ($e:expr) => {
        #[allow()] $e
    };
}

fn main() {
    let _ = #[allow()] 1 + 1;
    let _ = group!(#[allow()] 1) + 1;
    let _ = 1 + group!(#[allow()] 1);
    let _ = extra!({ 0 }) + 1;
    let _ = extra!({ 0 } + 1);
}

let _ = #[allow()] 1 + 1;
let _ = #[allow()] 1 + 1;
let _ = 1 + #[allow()] 1;
let _ = #[allow()] { 0 } + 1;
let _ = #[allow()] { 0 } + 1;

The first 4 statements are the correct expansion, but the last one is not. The attribute is supposed to apply to the entire binary operation, not only to the left operand.

After this PR, the 5th statement will expand to:

let _ = #[allow()] ({ 0 } + 1);

In the future, as some subset of stmt_expr_attributes approaches stabilization, it is possible that we will need to do parenthesization for a number of additional cases depending on the outcome of #127436. But for now, at least this PR makes the pretty-printer align with the current behavior of the parser.

r? fmease