rustc test annotations

Tests in https://github.com/rust-lang/rust can be linked to rules in the Reference. The rule will include a link to the tests, and there is also an appendix that tracks how the rules are currently linked.

Tests in the tests directory can be annotated with the //@ reference: x.y.z header to link them to a rule. The header can be specified multiple times if a single file covers multiple rules.

Compiler developers are not expected to add reference annotations to tests. However, if they do want to help, their cooperation is welcome. Reference authors and editors are responsible for ensuring every rule has a test associated with it.

The tests are beneficial for reviewers to see the behavior of a rule. They are also a benefit to readers who may want to see examples of particular behaviors. When adding new rules, you should wait until the Reference side is approved before submitting a PR to rust-lang/rust (to avoid churn if we decide on different names).

Always annotate with the most specific rule name available. For example, use asm.rules.reg-not-input rather than the broader asm.rules.

Complete coverage is the goal but is not yet expected.