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
https://github.com/rust-lang/rust/pull/73594#issuecomment-647671463)

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.

0 files changed, 0 insertions, 0 deletions