[OpenCL] Add "cles_khr_int64" extension.

Summary:
For OpenCL 1.1 embedded profile 64 bit integers i.e. long,
ulong including the appropriate vector data types and operations
on 64-bit integers are optional. The "cles_khr_int64" extension
string will be reported if the embedded profile implementation
supports 64-bit integers.

Reviewers: Anastasia, bader

Reviewed By: Anastasia, bader

Subscribers: bader, yaxunl, Anastasia, cfe-commits

Differential Revision: https://reviews.llvm.org/D42532

llvm-svn: 323522