[clang] [llvm] [clang][WebAssembly] Fix Wasm Vararg pointer width (PR #173580)

[Folkert de Vries via cfe-commits]

folkertdev wrote:

Just checking back in here, as we're moving towards stabilizing c-variadic functions in rust. 

Is the current c-variadic behavior on `wasm64` stable and, to the best of our knowledge, bugfree at this point?

https://github.com/llvm/llvm-project/pull/173580