Pull in proc-macro2 1.0.46 for byte order mark fix
Pull in proc-macro2 1.0.46 for byte order mark fix

Without this, our benchmarks fail against the current contents of the
rust-lang/rust repo.

    $ cargo test --release --bench rust --features full,test
    871050 lines in 12914 files
    read_from_disk:     elapsed=0.170s
    tokenstream_parse:  FAIL tests/rust/src/test/ui/json/json-bom-plus-crlf-multifile-aux.rs
    thread 'main' panicked at 'assertion failed: `(left == right)`
      left: `12913`,
     right: `12914`', benches/rust.rs:122:5
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

dtolnay/syn