[gen.rs] Improve `cenum_value_to_int_lit`.
[gen.rs] Improve `cenum_value_to_int_lit`.

alexander255/rust-bindgen,alexander255/rust-bindgen,Twinklebear/rust-bindgen,bsteinb/rust-bindgen,bsteinb/rust-bindgen,bsteinb/rust-bindgen,Twinklebear/rust-bindgen,Twinklebear/rust-bindgen,alexander255/rust-bindgen,Twinklebear/rust-bindgen