Rollup merge of #124051 - dtolnay:emptyset, r=compiler-errors

Fix empty-set symbol in comments

The symbol in the original code is U+00D8 "LATIN CAPITAL LETTER O WITH STROKE" (https://en.wikipedia.org/wiki/%C3%98) which is an uppercase letter in Danish, Norwegian, Faroese, and Southern Sámi alphabets.

The symbol that was intended is U+2205 "EMPTY SET" (https://en.wikipedia.org/wiki/Empty_set#Notation).

| Before | After |
|---|---|
| ![Screenshot from 2024-04-16 18-25-01](https://github.com/rust-lang/rust/assets/1940490/9b8b0624-cfa5-4b89-84e5-4c2b39c2cb8f) | ![Screenshot from 2024-04-16 18-25-05](https://github.com/rust-lang/rust/assets/1940490/6f6b34c3-0e47-4ba0-856d-be1dc164c94c) |