_Atomic crashes bindgen even when blocklisted

[Kriskras99]

Input C/C++ Header

_Atomic(int) a;
int b;

Bindgen Invocation

$ bindgen --blocklist-item "a" input.h

Actual Results

$ RUST_BACKTRACE=1 bindgen --blocklist-item "a" input.h
panicked at /home/kriskras99/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bindgen-0.70.1/ir/var.rs:319:25:
Couldn't resolve constant type, and it wasn't an nondeductible auto type or unexposed type!
panicked at core/src/panicking.rs:221:5:
panic in a function that cannot unwind
thread caused non-unwinding panic. aborting.
[1]    21518 IOT instruction (core dumped)  RUST_BACKTRACE=1 bindgen --blocklist-item "a" input.h

Expected Results

I expect bindgen to ignore the _Atomic type as it is on a blocklist:

/* automatically generated by rust-bindgen 0.70.1 */

extern "C" {
    pub static mut b: ::std::os::raw::c_int;
}

In addition, I think the panic message is confusing and should be more like "_Atomic types are not supported" or something similar.

[Kriskras99]

Potential solution could be generating _Atomic types as compile_error!("_Atomic types are not supported by bindgen").
This would give a clear error message to users (instead of Couldn't resolve constant type, ...) and would allow blocklist_.* and opaque_type to remove it.
Downside is that this would push the error to the Rust compilation step instead of the Bindgen step.

[emilio]

We should probably just generate the underlying type for now. Maybe we should do something smarter but rust doesn't have generic atomics so it's going to be a bit of a whack-a-mole.

[emilio]

#2921 has an initial pass at not crashing here at least.