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

[via cfe-commits]

LuoYuanke wrote:

> I'd expect this to completely remove usage of `CUDAAllowVariadicFunctions` and remove the associated driver handling code. That way passing it will result in the generic 'argument unused' warning. Though, I don't know if there was anything special about the old handling, the 'new' handling is ABI compatible with it, so I'd assume not but just in case I'll refer to @Artem-B

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

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