Add "reference" as a known compiletest header

This adds the "reference" compiletest header so that the Rust reference
can add annotations to the test suite in order to link tests to
individual rules in the reference.

Tooling in the reference repo will be responsible for collecting these
annotations and linking to the tests.

More details are in MCP 783: rust-lang/compiler-team#783

Author: ehuss

src/tools/compiletest/src/command-list.rs

@@ -208,6 +208,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "pretty-compare-only",
     "pretty-expanded",
     "pretty-mode",
+    "reference",
     "regex-error-pattern",
     "remap-src-base",
     "revisions",