Insert parentheses around binary operation with attribute
Fixes the bug found by `@fmease` in https://github.com/rust-lang/rust/pull/134661#pullrequestreview-2538983253.
Previously, `-Zunpretty=expanded` would expand this program as follows:
```rust
#![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);
}
```
```console
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:
```console
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 https://github.com/rust-lang/rust/issues/127436. But for now, at least this PR makes the pretty-printer align with the current behavior of the parser.
r? fmease
8751016e20