Rollup merge of #133730 - dtolnay:parentest, r=compiler-errors

Add pretty-printer parenthesis insertion test

This test demonstrates numerous bugs in rustc_ast_pretty, including all five of:

- Failing to insert parentheses where necessary to preserve the meaning of a syntax tree, producing invalid syntax.
- Failing to insert parentheses, producing valid syntax with the wrong meaning.
- Inserting too many parentheses.
- Inserting parentheses in the wrong place, producing invalid syntax.
- Losing syntactically significant parts of the syntax tree.

These pretty-printer bugs have consequences for `-Zunpretty=expanded`. The `cargo expand` subcommand cannot work reliably unless rustc can consistently produce valid Rust output. Erroneous syntax cannot be passed through rustfmt, or queried with [syn-select](https://crates.io/crates/syn-select).

The test in this PR is a port of a test from Syn that tests the automatic parenthesis insertion performed by Syn's `ToTokens` impls. In Syn we actually run this test over every expression in every Rust source file in the whole rust-lang/rust repo, including rustc and the standard library and tools and test suites. For the test here, I have only used a small selection of interesting expressions. This will serve as an easy spot to accumulate regression tests as the various bugs get fixed. Once rustc's pretty-printer is in better shape, it's possible that the test can be expanded to cover a larger set of expressions collected automatically like in Syn.

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