A recent patch changed LLVM to use an LLVM_CXX_STD variable for
defining the C++ standard to use and also removed the default unset
case from HandleLLVMOption's usage. This broke standalone builds since,
for example, lldb's LLDBStandalone.cmake called HandleLLVMOptions.cmake
without this variable set.
Define a default value of c++11 for this value.