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

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

This seems legitimate to me. The IR pass is enabled on nvptx and turns LLVM variadics into functions which match the PTX variadic calling convention as far as I know. I _think_ the guard is still there on CUDA because Joseph and I were mostly thinking in terms of freestanding C++ when we wrote this.

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