[OpenCL] Allow explicit cast of 0 to event_t.

Patch by Aaron Enye Shi.

Differential Revision: http://reviews.llvm.org/D17578

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270238 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed