Failure to evaluate #define directives if they use values from defines passed through clang args

It seems that while bindgen respects preprocessor defines passed through clang for evaluating ifdefs, it does not consider them for processing #define directives.

While the obvious workaround (or rather perhaps, the preferred way) is to put these defines in a wrapper.h header instead of passing them as arguments to clang, I stumbled over this after thinking that I do not need a wapper.

Regardless, it might be a good idea to mention this in Create a wrapper.h Header.

Input C/C++ Header

#define FOO_A 7
#ifdef FOO_FROM_CLANG
#define FOO_B 3
#endif
#define FOO_C (FOO_FROM_CLANG)
#define FOO_D (FOO_FROM_CLANG + 1)

Bindgen Invocation

$ bindgen input.h -- -DFOO_FROM_CLANG=3

Actual Results

/* automatically generated by rust-bindgen 0.63.0 */

pub const FOO_A: u32 = 7;
pub const FOO_B: u32 = 3;

Expected Results

All defines based off defines provided through clang show up in the generated bindings:

/* automatically generated by rust-bindgen 0.63.0 */

pub const FOO_A: u32 = 7;
pub const FOO_B: u32 = 3;
pub const FOO_C: u32 = 7;
pub const FOO_D: u32 = 8;

Reproduce the issue with input.h and bindgen input.h -- -DFOO_FROM_CLANG=3, comparing the generated values for FOO_A through FOO_D. Trace how bindgen processes clang-provided defines versus #define directives; done means FOO_C and FOO_D appear with the expected evaluated values, with coverage for this invocation.