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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

If there's no va_start and we're optimising there's no work to do. Otherwise when lowering we still transform declarations (which have no basic blocks) and calls to unknown pointers, so that separate compilation works. That's why knowing it's the whole program would be useful.

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