[rustc_data_structures][perf] Simplify base_n::push_str.

This minor change removes the need to reverse resulting digits.
Since reverse is O(|digit_num|) but bounded by 128, it's unlikely
to be a noticeable in practice. At the same time, this code is
also a 1 line shorter, so combined with tiny perf win, why not?

I ran https://gist.github.com/ttsugriy/ed14860ef597ab315d4129d5f8adb191
on M1 macbook air and got a small improvement
```
Running benches/base_n_benchmark.rs (target/release/deps/base_n_benchmark-825fe5895b5c2693)
push_str/old            time:   [14.180 µs 14.313 µs 14.462 µs]
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  4 (4.00%) high mild
  1 (1.00%) high severe
push_str/new            time:   [13.741 µs 13.839 µs 13.973 µs]
                        Performance has improved.
Found 8 outliers among 100 measurements (8.00%)
  3 (3.00%) high mild
  5 (5.00%) high severe
```

Author: ttsugriy

compiler/rustc_data_structures/src/base_n.rs

@@ -16,22 +16,21 @@ const BASE_64: &[u8; MAX_BASE] =
 pub fn push_str(mut n: u128, base: usize, output: &mut String) {
     debug_assert!(base >= 2 && base <= MAX_BASE);
     let mut s = [0u8; 128];
-    let mut index = 0;
+    let mut index = s.len();
 
     let base = base as u128;
 
     loop {
+        index -= 1;
         s[index] = BASE_64[(n % base) as usize];
-        index += 1;
         n /= base;
 
         if n == 0 {
             break;
         }
     }
-    s[0..index].reverse();
 
-    output.push_str(str::from_utf8(&s[0..index]).unwrap());
+    output.push_str(str::from_utf8(&s[index..]).unwrap());
 }
 
 #[inline]