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 })
```

compiler/rustc_expand/src/build.rs

@@ -272,6 +272,10 @@ impl<'a> ExtCtxt<'a> {
         self.expr(sp, ast::ExprKind::AddrOf(ast::BorrowKind::Ref, ast::Mutability::Not, e))
     }
 
+    pub fn expr_paren(&self, sp: Span, e: P<ast::Expr>) -> P<ast::Expr> {
+        self.expr(sp, ast::ExprKind::Paren(e))
+    }
+
     pub fn expr_call(
         &self,
         span: Span,