[clang] [X86_32][C++] fix 0 sized struct case in vaarg. (PR #86388)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

There are two ways we could analyze, this, I think:

- EmitVAArg is doing something which disagrees with the classification done by classifyArgumentType, so that's a bug in EmitVAArg.
- classifyArgumentType is returning a malformed value; EmitVAArg should assert that it never sees such a value, and classifyArgumentType should be changed to return something else in that case.

What exactly is classifyArgumentType returning here?

https://github.com/llvm/llvm-project/pull/86388