Silence g++ 4.9 build issue

lib/ASTMatchers/ASTMatchFinder.cpp:276:28: error: typedef 'traverse_can_only_be_instantiated_with_base_type' locally defined but not used [-Werror=unused-local-typedefs]
                            traverse_can_only_be_instantiated_with_base_type);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198256 91177308-0d34-0410-b5e6-96231b3b80d8