[OpenCL] Add clang extension for variadic functions.

With the internal clang extension '__cl_clang_variadic_functions'
variadic functions are accepted by the frontend.

This is not a fully supported vendor/Khronos extension
as it can only be used on targets with variadic prototype
support or in metaprogramming to represent functions with
generic prototype without calling such functions in the
kernel code.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D94027