When we hit an accept state in either the DFA or the ATN, we
have to notify the character stream to start buffering characters
via
IntStream.mark() and record the current state. The current sim state
includes the current index into the input, the current line,
and current character position in that line. Note that the Lexer is
tracking the starting line and characterization of the token. These
variables track the "state" of the simulator when it hits an accept state.
We track these variables separately for the DFA and ATN simulation
because the DFA simulation often has to fail over to the ATN
simulation. If the ATN simulation fails, we need the DFA to fall
back to its previously accepted state, if any. If the ATN succeeds,
then the ATN does the accept and the DFA simulator that invoked it
can simply return the predicted token type.