[OpenCL] Improved diagnostics of OpenCL types.

 - Changes diagnostics for Blocks to be implicitly
const qualified OpenCL v2.0 s6.12.5.

 - Added and unified diagnostics of some OpenCL special types:
blocks, images, samplers, pipes. These types are intended for use
with the OpenCL builtin functions only and, therefore, most regular
uses are not allowed including assignments, arithmetic operations,
pointer dereferencing, etc.

Review: http://reviews.llvm.org/D21989



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275061 91177308-0d34-0410-b5e6-96231b3b80d8