[Upstream] Handle sample image types.

[bader]

#1945 and f9520e6 added new built-in types, which are used in DPC++ library headers.

We should upstream this patch to llorg to enable parsing of DPC++ headers.

This issue blocks https://github.com/intel/llvm/milestone/1. To unblock this milestone we could guard sampled image types uses in DPC++ with pre-process for compilers w/o sampled image types support.