[clang] [CUDA] Remove sema check of function declaration with variadic argument (PR #161350)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> If we completely remove `CUDAAllowVariadicFunctions`, I'm afraid it may break compiling legacy code which is specified the `-fcuda-allow-variadic-functions` option.

The driver and `cc1` flag would still exist, it would just be completely unused. The only concern here would be if the old handling was ABI compatible, but I think all the old handling did was suppress the warning to allow emitting variadic function signatures or something.

https://github.com/llvm/llvm-project/pull/161350