Auto merge of #147799 - Enselic:slash-slash-at-debuginfo-tests, r=saethlin

compiletest: Use `//@` prefixes also for debuginfo test directives

So that when we later add support for revisions we can use the same syntax for revisions as elsewhere (for rust-lang/rust#147426).

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 add one FIXME for a non-trivial change for later:
```
// FIXME(#148097): Change `// cdb-checksimple_closure` to `//@ cdb-check:simple_closure`
```

### TODO
- [x] Triple-check that all tests still run and all directives are still applied. Done: rust-lang/rust#147799 (comment)

### Zulip discussion

https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/.2F.2F.40.20syntax.20for.20debuginfo.20tests/with/545015582

Author: bors