Stop normalizing so many different prefixes

Previously, we would normalize *all* of
- the absolute path to the repository checkout
- the /rustc/$sha for stage1 (if `remap-debuginfo` was enabled)
- the /rustc/$sha for download-rustc
- the sysroot for download-rustc

Now, we consistently only normalize /rustc/FAKE_PREFIX. Not only is this
much simpler, but it also avoids ongoing maintenance for download-rustc
and makes it much less likely that tests break by accident.

- Change `tests/ui/track-diagnostics/track6.rs` to use a relative path
  instead of an absolute one. I am not actually sure why `track_caller`
works here, but it does seem to work 🤷

- Pass `-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX` to all
  suites, not just UI. In particular, mir-opt tests emit /rustc/ paths
  in their output.

compiler/rustc_session/src/parse.rs

@@ -84,6 +84,7 @@ impl SymbolGallery {
 
 /// Construct a diagnostic for a language feature error due to the given `span`.
 /// The `feature`'s `Symbol` is the one you used in `active.rs` and `rustc_span::symbols`.
+#[track_caller]
 pub fn feature_err(
     sess: &ParseSess,
     feature: Symbol,