Bring order to the OpenCL keywords

Avoid the gratuitous repurposing of C++ keyword 'private' by using a keyword
alias.

Also attempt to document the OpenCL keywords based on scraps of information
found online.

The purpose of this commit is to reduce impact on the C++ parser.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197511 91177308-0d34-0410-b5e6-96231b3b80d8