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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

Early exit on lack of va_start will be incorrect in the lowering case, which is the only one enabled by default. I believe existing comments are all addressed. Precommit the cmake diagnostic tweak sounds good, would you like to land that?

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