[OpenCL] Generate metadata for opencl_unroll_hint attribute

Add support for opencl_unroll_hint attribute from OpenCL v2.0 s6.11.5.

Reusing most of metadata generation from CGLoopInfo helper class.

The code is based on Khronos OpenCL compiler:
https://github.com/KhronosGroup/SPIR/tree/spirv-1.0

Patch by Liu Yaxun (Sam)!

Differential Revision: http://reviews.llvm.org/D16686

llvm-svn: 261350