Bindgen fails to generate constants if `#define`s are out of order

[roypat]

Input C/C++ Header

#define A B
#define B 1

Bindgen Invocation

bindgen input.h

Actual Results

/* automatically generated by rust-bindgen 0.66.1 */

pub const B: u32 = 1;

Expected Results

I expected

pub const A: u32 = 1;
pub const B: u32 = 1;

Both gcc and clang accept these kind of "out of order" definitions (as counter-intuitive as it is), and the linux kernel makes use of this in kvm.h, which is how we discovered this issue in rust-vmm/kvm-bindings#89

[pvdrz]

I think #2561 doesn't strictly fix this but it is a step in that direction iiuc (cc @reitermarkus).

If that's the case maybe this is a good enough reason to do this change, what do you think @emilio?

[reitermarkus]

No, #2561 doesn't fix this, but #2369 which depends on #2561 will fix it.