Auto merge of #120881 - jieyouxu:migrate-ui-test-directives, r=oli-obk

Migrate ui tests from legacy compiletest-style directives `//` to `ui_test`-style directives `//@`

## Preface

There's an on-going effort to rewrite parts of or the entirety of compiletest
(<https://github.com/rust-lang/compiler-team/issues/536>). A step towards this involve migrating
ui tests to use the [`ui_test`](https://github.com/oli-obk/ui_test) framework, which involves
changing compiletest directives in `// <directive-name>` style to `ui_test` `//@ <directive-name>`
style (https://github.com/rust-lang/compiler-team/issues/512).

This PR aims to implement the directive-style change from `//` to `//`@`` for ui tests only and
make compiletest only accept `//`@`` directives in the "ui" test suite (only).

## Key Changes

1. All ui test `//` directives are replaced by `//`@`` directives.
2. Only accept `//`@`` directives for "ui" test suite.
3. Errors if a comment could be interpreted as a legacy-style `//` directive.

## Diff Generation

The diff is generated by:

- Collecting directives from ui tests via hacking on compiletest.
- Using a migration tool to replace `//` directives in ui tests with `//`@`.`

### Reproduction Steps

0. Delete the temporary directory `$RUSTC_REPO_PATH/build/x86_64-apple-darwin/test/ui/__directive_lines` and the temporary file `$RUSTC_REPO_PATH/build/x86_64-apple-darwin/test/ui/__directive_lines.txt` (if you ran the collection script before).
1. Use the <https://github.com/jieyouxu/rust/tree/collect-test-directives> collect-test-directives
   script, which outputs temporary files recording headers occuring in each ui test.
   - You need to checkout this branch: `git checkout collect-test-directives`.
   - You might need to rebase on lastest master and ensure there are no conflicts.
   - You likely need to run `./x test tests/ui --stage 1 --force-rerun` to generate the temporary
     files consistently.
2. Checkout the `migrate-ui-test-directives` branch.
4. Run the migration tool <https://github.com/jieyouxu/compiletest-ui_test-header-migration>.
    - You will need to first generate a `migration_config.toml` via `cargo run -- generate-config` under `$CWD`.
    - Then, update `manual_directives = ["// should-fail"]` in `migration_config.toml`. This is required because the collection script doesn't deal with some special meta ui tests and there are no other `// should-fail` occurrences.
5. Check that the migration at least does not cause UI test failures if you change compiletest to
   accept `//`@`` directives for ui tests only.
   - `RUSTC_TEST_FAIL_FAST=1 ./x test tests/ui --stage 1 --bless`
6. Confirm that there is no difference after running the migration tool when you are on the
   `migrate-ui-test-directives` branch.

## Next Steps

- [x] ~~Need to implement some kind of warning or tidy script to help contributors catch old-style
  `// <directive-name>` directives, while only accepting `ui_test`-style `//@ <directive-name>`
  directives.~~ An error is emitted if a comment that could be interpreted as legacy-style test directive is encountered.
- [ ] Need to properly document this change in e.g. rustc-dev-guide (https://github.com/rust-lang/rustc-dev-guide/pull/1885).
    - [x] Add a `README.md` to `tests/ui` describing the directive style change.

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