Support `#undef`

[ojeda]

From #3409 (comment) and #2722 (comment).

---

bindgen currently ignores #undef, e.g.

#define A 1
#undef A

generates

pub const A: u32 = 1;

but A should not exist.

This seems to be acknowledged in:

rust-bindgen/bindgen/ir/var.rs

Lines 491 to 496 in 25b2347

	// If I understood correctly, `bindgen` ignores all `#undef` directives. Meaning that this:
	// ```c
	// #define FOO 1
	// #undef FOO
	// #define FOO 2
	// ```