[OpenCL] Define CLK_NULL_EVENT without cast

Defining CLK_NULL_EVENT with a `(void*)` cast has the (unintended?)
side-effect that the address space will be fixed (as generic in OpenCL
2.0 mode).  The consequence is that any target specific address space
for the clk_event_t type will not be applied.

It is not clear why the void pointer cast was needed in the first
place, and it seems we can do without it.

Differential Revision: https://reviews.llvm.org/D63876

llvm-svn: 366546