Put helper classes into anonymous namespace.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185295 91177308-0d34-0410-b5e6-96231b3b80d8
Make string pointer const.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185296 91177308-0d34-0410-b5e6-96231b3b80d8