[clang] [CIR] Upstream __builtin_va_start and __builtin_va_end (PR #153819)

[Morris Hafner via cfe-commits]

mmha wrote:

I changed the op definition to keep the count in CIR. So I'm asking for a small re-review :)

It leads to somewhat worse LLVM IR because of a dead load but I don't think there's much we can do here.

https://github.com/llvm/llvm-project/pull/153819