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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Ah yes, libc code doing the equivalent of va_arg assuming natural alignment when the underlying buffer is a packed struct with fields padded to four bytes would not work. That would be "fixed" by changing the compiler to match the assumption made by libc, but it seems much better for libc to do the misaligned load instead. Then there's no wavesize*align-padding stack space burned at runtime.

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