[clang] [clang][Sema] Make passing a non-POD type to `va_arg` an error, not a warning (PR #198668)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Many many apologies: All my last comments were nonsense, it turns out I just can't read assembly any more. Many many apologies for the noise. I think this is fine, and it should be considered an assertion fix.

https://github.com/llvm/llvm-project/pull/198668