[OpenCL] opencl-c.h: remove arg names for image builtins

This simplifies completeness comparisons against OpenCLBuiltins.td and
also makes the header no longer "claim" the identifiers "image",
"image_array", "coord", "sampler", "sample", "gradientX", "gradientY",
"lod", and "color".

Continues the direction set out in D119560.