Add [TRACKED_NO_CRATE_HASH] and [SUBSTRUCT] directives

This is necessary for options that should invalidate the incremental
hash but *not* affect the crate hash (e.g. --remap-path-prefix).

This doesn't add `for_crate_hash` to the trait directly because it's not
relevant for *types*, only for *options*, which are fields on a larger
struct. Instead, it adds a new `SUBSTRUCT` directive for options, which
does take a `for_crate_hash` parameter.

- Use TRACKED_NO_CRATE_HASH for --remap-path-prefix
- Add test that `remap_path_prefix` is tracked
- Reduce duplication in the test suite to avoid future churn

compiler/rustc_incremental/src/persist/load.rs

@@ -104,7 +104,7 @@ pub fn load_dep_graph(sess: &Session) -> DepGraphFuture {
     // Fortunately, we just checked that this isn't the case.
     let path = dep_graph_path_from(&sess.incr_comp_session_dir());
     let report_incremental_info = sess.opts.debugging_opts.incremental_info;
-    let expected_hash = sess.opts.dep_tracking_hash();
+    let expected_hash = sess.opts.dep_tracking_hash(false);
 
     let mut prev_work_products = FxHashMap::default();
     let nightly_build = sess.is_nightly_build();

compiler/rustc_incremental/src/persist/save.rs

@@ -219,7 +219,7 @@ pub fn build_dep_graph(
     }
 
     // First encode the commandline arguments hash
-    if let Err(err) = sess.opts.dep_tracking_hash().encode(&mut encoder) {
+    if let Err(err) = sess.opts.dep_tracking_hash(false).encode(&mut encoder) {
         sess.err(&format!(
             "failed to write dependency graph hash `{}`: {}",
             path_buf.display(),