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.
Martin Nordholts