[OpenCL] Fix invalid address space generation for clk_event_t

Summary:
Addrspace(32) was generated when putting 0 in clk_event_t * event_ret
parameter for enqueue_kernel function.

Patch by Viktoria Maksimova

Reviewers: Anastasia, yaxunl, AlexeySotkin

Reviewed By:  Anastasia, AlexeySotkin

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53809

llvm-svn: 346838