So that when we later add support for revisions we can use the same
syntax for revisions as elsewhere.
This also prevents people from making typos for commands since
`src/tools/compiletest/src/directives/directive_names.rs` will catch such
typos now.
Note that we one FIXME for a non-trivial change for later:
```
// FIXME(148097): Change `// cdb-checksimple_closure` to `//@ cdb-check:simple_closure`
```
Combining revisions with only-arch allows specifying
that a test only applies to a handful of targets.
This allows removing a large amount of repetition
in the test suite for tests that do not benefit.
The revisions are suboptimal for this for some tests,
so they aren't preferred in those cases.
