[clang] [libc] [llvm] [NVPTX] Implement variadic functions using IR lowering (PR #96015)

[Tom Honermann via cfe-commits]

tahonermann wrote:

For historical reference, @jhuber6 backed out the change to `NVPTXTargetInfo::getBuiltinVaListKind()` in https://github.com/llvm/llvm-project/pull/100438 (commit https://github.com/llvm/llvm-project/commit/fb1e077982e90e715058188fa667ea5dd8027a98)

https://github.com/llvm/llvm-project/pull/96015