C++: Applied some C++11 refactorings to the runtime library and compiler

I basically used clang-tidy to apply the following C++11 refactorings (which make very much sense to me)

• make use of override keyword whenever a virtual function is overwritten
• make use of auto keyword for iterators
• make use of auto keyword when every a redundancy can be avoided, e.g. 
  uint32_t len = static_cast<uint32_t>(str.length());
  auto len = static_cast<uint32_t>(str.length());
• replaced NULL with nullptr
• make use of explicitly-defaulted function definition

Additionally, I applied some more const-correctness to some functions.