Replace remaining "C-like enumerations" with "fieldless enumerations"

[alercah]

See rust-lang/rust#46348. Looks like a few snuck through: https://doc.rust-lang.org/nightly/reference/type-layout.html#reprc-enums, for instance.

[Havvy]

Already done in my branch for the #[repr(C, Int)] on enums with fields RFC.