Implement llvm::raw_ostream::operator<< support for seec::cm::GlobalVariable,
and move the printing responsibility of global variable details out of the
printing logic for seec::cm::ProcessState.
Add a method to seec::cm::GlobalVariable to get the current seec::cm::Value for
the global variable.

When printing a seec::cm::GlobalVariable to a llvm::raw_ostream, also print
the current value (if available).