[OpenCL] Add image type handling for builtins

Image types were previously available, but not working.  This patch
adds image type handling.

Rename the image type definitions in the .td file to make them
consistent with other type names.  Use abstract types to represent the
unqualified types.  Instantiate access-qualified image types at the
point of use using, e.g. `ImageType<Image2d, "RO">`.

Add/update TableGen definitions for the read_image/write_image
builtin functions.

Patch by Pierre Gondois and Sven van Haastregt.

Differential Revision: https://reviews.llvm.org/D63480

llvm-svn: 371046