Invalid c struct bitfield generation in some cases

[Strackeror]

Discussed in #3104

So, i'm trying to run bindgen from a pokemon decomp project, and having issues with some structs not passing the sanity checks.

I've pinpointed it to an alignment issue on some structs with bitfields, here is a minimal reproduction:

struct Test
{
    char a;
    char b:4;
    char c:4; 

    short x:6;
    short y:10;
};

clang/gcc make that struct fit into 4 bytes, but bindgen adds a full byte of padding between c and x

I've experimented quickly and stepped through the code for handling bitfields in bindgen/ir/comp.rs and I'm pretty sure it's an issue with bindgen's handling. I think it's because bitfield groups are constructed as if they are already starting aligned, but I'm an informed layman at best.

[emilio]

Yeah this looks like a bug. Unfortunately our bitfield generation is kinda reverse-engineered from LLVM, it seems we're getting that case wrong. Fixing and adding it as a test would be great.

[emilio]

This might be a dupe of #1377