[clang] [llvm] [LLVM] Fix incorrect alignment on AMDGPU variadics (PR #96370)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > > Incrementing by align is just a bug, of course the size is the real value. Whether we want to continue wasting space is another not-correctness discussion
> > 
> > 
> > Struct padding is pretty universal, AMDGPU seems the odd one out here. I wouldn't mind it so much if it didn't require me to know which vendor I was dealing with in the RPC implementation, but I suppose I could store that information somewhere if we want to use a compressed option and we know it works.
> 
> It's not about struct padding, but the base alignment. Any pointer increment should be alignTo(ptr + size, align), not += align. The += align won't even work for large structs

Hm, that's what I'm doing in the `printf` implementation and it doesn't work without that patch. When I look at the varargs struct it didn't have any padding, which explained why `alignTo(ptr + size, align)` was wrong. So, I was trying to do the following, `printf("%d%ld", 1, 1l)`. With this patch I get the following,
```
0xbebebebe00000001
0x0000000000000001
```
Without this patch, I get this. As you can see there's no struct padding so the 8 byte value is right next to the 4 byte one.
```
0x0000000100000001
0xbebebebe00000000
```

https://github.com/llvm/llvm-project/pull/96370