[clang] [clang][CodeGen] Return RValue from `EmitVAArg` (PR #94635)

[John McCall via cfe-commits]

https://github.com/rjmccall commented:

Other than the one slight nit (that I think may be irrelevant), this LGTM.  I'd like @efriedma-quic to also do a quick review if he has time, though.

https://github.com/llvm/llvm-project/pull/94635