[clang] [Clang] Remove use of deprecated `-fucda-allow-variadic-functions` (PR #179724)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Didn't you just enable variadic function codegen, so this works now?

Yeah this was done awhile back in https://github.com/llvm/llvm-project/pull/161305. The flag was listed as deprecated but the uses weren't removed.

https://github.com/llvm/llvm-project/pull/179724