[clang] [libc] [llvm] [AMDGPU] Implement variadic functions by IR lowering (PR #93362)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

We can check for the va_start which is a good approximation, but there can be a variadic function pointer from another module and the only place that would show up is a call instruction, and the only way to find that is enumeration. Amdgpu can somewhat cheat using the whole.program assumption but it's sketchy.

https://github.com/llvm/llvm-project/pull/93362