Fix proc macro punctuation panic

[fw-immunant]

Should fix #1142.

[kkysen]

LGTM.

@kornelski, does this fix the issue? You can test it by checking out this branch and then rm Cargo.lock and cargo +stable install --path c2rust (since cargo install --git doesn't allow --bin).

[fw-immunant]

LGTM.

@kornelski, does this fix the issue? You can test it by checking out this branch and then rm Cargo.lock and cargo +stable install --path c2rust (since cargo install --git doesn't allow --bin).

This appears to fix the issue locally; a build with proc-macro2 v1.0.93 following the above instructions seems to correctly process the .c files I threw at it.