[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

llvm-svn: 276161