`ExprNode` and its child classes use LLVM-style RTTI.
Until now their destruction was not handled properly, causing the
constructor-destructor type mismatch warnings on ASAN.
Despite this all the code was working properly, but just for luck,
because of the fact that these classes are very shallow.
This commit fixes the issue, and allow `ExprNode` and its child classes
to be extended without worrying about wrong destructors being invoked.
Pietro Fezzardi