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
Anastasia Stulova