custom derives don't work on enums

[ericseppanen]

@trobanga pointed out that PR #2059 (implementing #1089) doesn't seem to work if we try to add a custom derive to an enum instead of a struct.

I can see the problem if I try to extend the unit test from #2059 like this:

Input C/C++ Header

enum my_rustified_enum {
  ELEVEN = 11,
  TWELVE,
  THIRTEEN,
};

#[test]
fn test_custom_derive() {
    use std::cmp::{PartialEq, Ordering};

    let test3 = unsafe { bindings::my_rustified_enum::ELEVEN };
    let test4 = unsafe { bindings::my_rustified_enum::TWELVE };

    assert!(test3 < test4);
}

Bindgen Invocation

I tested this using cargo test within bindgen-integration/, with the custom derives specified like this:

fn add_derives(&self, name: &str) -> Vec<String> {
    if name == "my_rustified_enum" {
        vec!["PartialOrd".into()]
    } else {
        vec![]
    }
}

Actual Results

$ cargo test
   Compiling bindgen-integration v0.1.0 (.../rust-bindgen/bindgen-integration)
error[E0369]: binary operation `<` cannot be applied to type `root::my_rustified_enum`
   --> src/lib.rs:289:19
    |
289 |     assert!(test3 < test4);
    |             ----- ^ ----- root::my_rustified_enum
    |             |
    |             root::my_rustified_enum
    |
    = note: an implementation of `std::cmp::PartialOrd` might be missing for `root::my_rustified_enum`

Expected Results

PartialOrd should have been added to the #[derive(...) attribute, allowing this test to compile.