Make real_rust_path_dir a TRACKED_NO_CRATE_HASH option

This also adds support for doc-comments to Options.

compiler/rustc_interface/src/tests.rs

@@ -459,6 +459,10 @@ fn test_top_level_options_tracked_no_crate() {
     // Make sure that changing a [TRACKED_NO_CRATE_HASH] option leaves the crate hash unchanged but changes the incremental hash.
     // This list is in alphabetical order.
     tracked!(remap_path_prefix, vec![("/home/bors/rust".into(), "src".into())]);
+    tracked!(
+        real_rust_source_base_dir,
+        Some("/home/bors/rust/.rustup/toolchains/nightly/lib/rustlib/src/rust".into())
+    );
 }
 
 #[test]