Stop using old version of syn in rustc-workspace-hack

None of the tools seem to need syn 0.15.35, so we can just build syn
1.0.

This was causing an issue with clippy's `compile-test` program: since
multiple versions of `syn` would exist in the build directory, we would
non-deterministically pick one based on filesystem iteration order. If
the pre-1.0 version of `syn` was picked, a strange build error would
occur (see
rust-lang/rust#73594 (comment))

To prevent this kind of issue from happening again, we now panic if we
find multiple versions of a crate in the build directly, instead of
silently picking the first version we find.

tests/compile-test.rs

@@ -49,7 +49,9 @@ fn third_party_crates() -> String {
         if let Some(name) = path.file_name().and_then(OsStr::to_str) {
             for dep in CRATES {
                 if name.starts_with(&format!("lib{}-", dep)) && name.ends_with(".rlib") {
-                    crates.entry(dep).or_insert(path);
+                    if let Some(old) = crates.insert(dep, path.clone()) {
+                        panic!("Found multiple rlibs for crate `{}`: `{:?}` and `{:?}", dep, old, path);
+                    }
                     break;
                 }
             }