[OpenCL] Diagnose write_only image3d when extension is disabled

Prior to OpenCL 2.0, image3d_t can only be used with the write_only
access qualifier when the cl_khr_3d_image_writes extension is enabled,
see e.g. OpenCL 1.1 s6.8b.

Require the extension for write_only image3d_t types and guard uses of
write_only image3d_t in the OpenCL header.

Patch by Sven van Haastregt!

Review: https://reviews.llvm.org/D28860
llvm-svn: 293050