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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

An offline suggestion from Pierre is that this should early-exit if there are no variadic functions in the module. That's a good thing, I'd like to consider it another of the increase-complexity-for-decreased-compile-time to implement after something has landed.

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