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

[Joseph Huber via cfe-commits]

jhuber6 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.

Figured if there's no `va_start` there's nothing for the pass to do anyway.

> Precommit the cmake diagnostic tweak sounds good, would you like to land that?

Sure

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