Simplifying the OpenCL image attribute. It does not need a semantic integer parameter because the required information is encoded in the spelling. Added an appropriate subject to the attribute, and simplified the semantic checking (which will likely be expanded upon in a future patch). Also, removed the GNU spelling since it was unsupported in the first place.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199229 91177308-0d34-0410-b5e6-96231b3b80d8