[OpenCL] Do not use vararg in emitted functions for enqueue_kernel

Not all targets support vararg (e.g. amdgpu). Instead of using vararg in the emitted functions for enqueue_kernel,
this patch creates a temporary array of size_t, stores the size arguments in the temporary array
and passes it to the emitted functions for enqueue_kernel.

Differential Revision: https://reviews.llvm.org/D36678

llvm-svn: 312441