[OpenCL] Make is_valid_event, create_user_event overloadable.

Summary: Make is_valid_event and create_user_event overloadable like other built-ins.

Patch by Evgeniy Tyurin.

Reviewers: bader, yaxunl

Subscribers: Anastasia, cfe-commits

Differential Revision: https://reviews.llvm.org/D23914

llvm-svn: 280097