Auto merge of #121370 - jieyouxu:migrate-compiletest-directives, r=oli-obk

Migrate compiletest to use `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
compiletest 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 the remaining
non-"ui" test suite tests.

## Key Changes

1. All `tests/` tests now use `//`@`` directives.
2. Compiletest only accepts `//`@`` and issues an error if an old-style directive is detected.
3. `// ignore-tidy` and `// ignore-tidy-*` are considered tidy directives and are ignored by
   compiletest header parsing.

## Diff Generation

The diff is generated by:

- Collecting directives from `tests/` via hijacking compiletest to emit successfully parsed
  directive lines.
- Using a migration tool
  (<https://github.com/jieyouxu/compiletest-ui_test-header-migration/tree/master>) to replace
  `//` directives in compiletest tests with `//`@`.`

### Reproduction Steps

0. Delete the temporary file `$RUSTC_REPO_PATH/build/<target_triple>/test/__directive_lines.txt`,
   if the collection script was previously ran.
1. Use the <https://github.com/jieyouxu/rust/tree/collect-test-directives> collect-test-directives
   script, which outputs a temporary file recording headers occuring in each compiletest test.
   - You need to checkout this branch: `git checkout collect-test-directives`.
   - This needs to be rebased on latest master to ensure up-to-date test directives can be collected.
   - You need to run `./x test` on each of the `test/*` subfolders once:

        ```bash
        ./x test tests/assembly/ --stage 1 --force-rerun
        ./x test tests/codegen/ --stage 1 --force-rerun
        ./x test tests/codegen-units/ --stage 1 --force-rerun
        ./x test tests/coverage/ --stage 1 --force-rerun
        ./x test tests/coverage-run-rustdoc/ --stage 1 --force-rerun
        ./x test tests/debuginfo/ --stage 1 --force-rerun
        ./x test tests/incremental/ --stage 1 --force-rerun
        ./x test tests/mir-opt/ --stage 1 --force-rerun
        ./x test tests/pretty/ --stage 1 --force-rerun
        ./x test tests/run-make/ --stage 1 --force-rerun
        ./x test tests/run-make-fulldeps/ --stage 1 --force-rerun
        ./x test tests/run-pass-valgrind/ --stage 1 --force-rerun
        ./x test tests/rustdoc/ --stage 1 --force-rerun
        TARGET=<target-triple> ./x test tests/rustdoc-gui/ --stage 1 --force-rerun
        ./x test tests/rustdoc-js/ --stage 1 --force-rerun
        ./x test tests/rustdoc-js-std/ --stage 1 --force-rerun
        ./x test tests/rustdoc-json/ --stage 1 --force-rerun
        ./x test tests/rustdoc-ui/ --stage 1 --force-rerun
        ./x test tests/ui/ --stage 1 --force-rerun
        ./x test tests/ui-fulldeps/ --stage 1 --force-rerun
        ```

2. Checkout the `migrate-compiletest-directives` branch.
3. Run the migration tool <https://github.com/jieyouxu/compiletest-ui_test-header-migration>.
4. Check that the migration at least does not cause test failures if you change compiletest to
   accept `//`@`` directives only. This is also required if the test outputs somehow need to be
   blessed.
   - `RUSTC_TEST_FAIL_FAST=1 ./x test tests/<secondary-directory>  --stage 1 --bless`
5. Confirm that there is no difference after running the migration tool when you are on the
   `migrate-compiletest-directives` branch.

## Follow Up Work

- [ ] Adjust rustc-dev-guide docs for compiletest directives (this time for all the other suites and modes). <https://github.com/rust-lang/rustc-dev-guide/pull/1895>.

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