This commit adds an intrinsic that will read from an image buffer. We
chose to match the name of the DXIL intrinsic for simplicity in clang.
We cannot reuse the existing openCL readimage function because that is
not a reserved name in HLSL.
I considered trying to refactor generateReadImageInst, so that we could
share code between the two implementations. However, most of the code in
generateReadImageInst is concerned with trying to figure out which type
of image read is being done. Once we factor out the code that will be
common, then we end up with just a single call to the MIRBuilder being
common.
