[CUDA] Relax restrictions on GPU-side variadic functions

Allow parsing GPU-side variadic functions when we're compiling with CUDA-9 or
newer. We still do not allow accessing variadic arguments.

CUDA-9 was the version which introduced PTX-6.0 which allows implementing
variadic functions, so older versions can't have variadics in principle.

This is required for dealing with headers in recent CUDA versions that rely on
variadic function declarations in some of the templated code in libcu++.
E.g. https://github.com/llvm/llvm-project/issues/58410

Differential Revision: https://reviews.llvm.org/D150718