Speculative build fix for r188206.

The cmake-clang-x86_64 was upset:
error: 'template<class ImplClass, class RetTy> class clang::ConstStmtVisitor' used without template parameters

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188211 91177308-0d34-0410-b5e6-96231b3b80d8