Rollup merge of #107488 - nnethercote:fix-PartialEq-syntax, r=RalfJung

Rollup merge of #107488 - nnethercote:fix-PartialEq-syntax, r=RalfJung

Fix syntax in `-Zunpretty-expanded` output for derived `PartialEq`.

If you do `derive(PartialEq)` on a packed struct, the output shown by `-Zunpretty=expanded` includes expressions like this:
```
{ self.x } == { other.x }
```
This is invalid syntax. This doesn't break compilation, because the AST nodes are constructed within the compiler. But it does mean anyone using `-Zunpretty=expanded` output as a guide for hand-written impls could get a nasty surprise.

This commit fixes things by instead using this form:
```
({ self.x }) == ({ other.x })
```

r? ``@RalfJung``