[OpenCL] AMDGCN target will generate images in constant address space
Allows AMDGCN target to generate images (such as %opencl.image2d_t) in constant address space.
Images will still be generated in global address space by default.
Added tests to existing opencl-types.cl in test\CodeGenOpenCL.
Patch by Aaron En Ye Shi.
Differential Revision: https://reviews.llvm.org/D22523