Creates or gets a Claripy solver, based on the state options.

def _solver(self):
        """
        Creates or gets a Claripy solver, based on the state options.
        """
        if self._stored_solver is not None:
            return self._stored_solver

        track = o.CONSTRAINT_TRACKING_IN_SOLVER in self.state.options
        approximate_first = o.APPROXIMATE_FIRST in self.state.options

        if o.STRINGS_ANALYSIS in self.state.options:
            if 'smtlib_cvc4' in backend_manager.backends._backends_by_name:
                our_backend = backend_manager.backends.smtlib_cvc4
            elif 'smtlib_z3' in backend_manager.backends._backends_by_name:
                our_backend = backend_manager.backends.smtlib_z3
            elif 'smtlib_abc' in backend_manager.backends._backends_by_name:
                our_backend = backend_manager.backends.smtlib_abc
            else:
                raise ValueError("Could not find suitable string solver!")
            if o.COMPOSITE_SOLVER in self.state.options:
                self._stored_solver = claripy.SolverComposite(
                    template_solver_string=claripy.SolverCompositeChild(backend=our_backend, track=track)
                )
        elif o.ABSTRACT_SOLVER in self.state.options:
            self._stored_solver = claripy.SolverVSA()
        elif o.SYMBOLIC in self.state.options and o.REPLACEMENT_SOLVER in self.state.options:
            self._stored_solver = claripy.SolverReplacement(auto_replace=False)
        elif o.SYMBOLIC in self.state.options and o.CACHELESS_SOLVER in self.state.options:
            self._stored_solver = claripy.SolverCacheless(track=track)
        elif o.SYMBOLIC in self.state.options and o.COMPOSITE_SOLVER in self.state.options:
            self._stored_solver = claripy.SolverComposite(track=track)
        elif o.SYMBOLIC in self.state.options and any(opt in self.state.options for opt in o.approximation):
            self._stored_solver = claripy.SolverHybrid(track=track, approximate_first=approximate_first)
        elif o.HYBRID_SOLVER in self.state.options:
            self._stored_solver = claripy.SolverHybrid(track=track, approximate_first=approximate_first)
        elif o.SYMBOLIC in self.state.options:
            self._stored_solver = claripy.Solver(track=track)
        else:
            self._stored_solver = claripy.SolverConcrete()

        return self._stored_solver