Pull in proc-macro2 1.0.46 for byte order mark fix

[dtolnay]

See dtolnay/proc-macro2#353.

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