Fix linker error in clang-fuzzer following 89ea0b05.

This fixes:
tools/clang/tools/clang-fuzzer/handle-cxx/CMakeFiles/obj.clangHandleCXX.dir/handle_cxx.cpp.o:handle_cxx.cpp:function clang_fuzzer::HandleCXX(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*, std::vector<char const*, std::allocator<char const*> > const&): error: undefined reference to 'clang::tooling::newInvocation(clang::DiagnosticsEngine*, llvm::SmallVector<char const*, 16u> const&)'