Move operator<<(llvm::raw_ostream &, UnicodeString) into the llvm namespace.
Add SeeC-Clang mapped state for runtime error states.

A vector of seec::cm::RuntimeErrorState objects is kept in the
seec::cm::FunctionState. The runtime error state objects refer to the unmapped
seec::trace::RuntimeErrorState, and provide convenience features such as finding
the clang::Stmt responsible for the error.