Install rustc_ast crate for the clippy CI job

    error[E0463]: can't find crate for `rustc_ast`
     --> tests/test_round_trip.rs:6:1
      |
    6 | extern crate rustc_ast;
      | ^^^^^^^^^^^^^^^^^^^^^^^ can't find crate
      |
      = help: maybe you need to install the missing components with: `rustup component add rust-src rustc-dev llvm-tools-preview`

.github/workflows/ci.yml

@@ -115,4 +115,6 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - uses: dtolnay/rust-toolchain@clippy
+        with:
+          components: rustc-dev
       - run: cargo clippy --all-features --tests -- -Dclippy::all -Dclippy::pedantic