Removing some attribute magic related to the OpenCL keyword attributes....

Removing some attribute magic related to the OpenCL keyword attributes. Instead of mapping them to their semantics as a custom part of the parser, they instead map declaratively through the rest of the attribute system.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199175 91177308-0d34-0410-b5e6-96231b3b80d8