Fix for antlr#2164.

[mike-lischke]

A different approach to the problem, now really fixing the leak.

[chrisaycock]

I hate to reopen this debate, but @chund is right; that is definitely a double free. In C++, throw copies the exception object and then invokes the destructor. So the catch object (in the generated parse() function from ...Parser.cpp) now contains a copy of the pointer that has just been freed. So once catch ends, the destructor is run a second time on that pointer.

The solution is to simply switch to a shared_ptr. This PR almost has it; there shouldn't be a destructor at all for NoViableAltException if _deadEndConfigs is a shared_ptr.

[mike-lischke]

@chund @chrisaycock since this PR is already closed let's discuss this issue in the reference bug report.