[OpenCL] Diagnose variadic arguments

OpenCL disallows using variadic arguments (s6.9.e and s6.12.5 OpenCL v2.0)
apart from some exceptions:
- printf
- enqueue_kernel

This change adds error diagnostic for variadic functions but accepts printf
and any compiler internal function (which should cover __enqueue_kernel_XXX cases).

It also unifies diagnostic with block prototype and adds missing uncaught cases for blocks.

llvm-svn: 285395