...generally, idlc leaks most memory, and e.g. trying to turn
AstExpression::m_exprValue into a unique_ptr would fail because
AstExpression::eval_symbol returns a pointer to an object "owned" elsewhere (but
never actually deleted by its owner).  So improvement of idlc ends here---for
one, it does not improve anything to try and make idlc not leak memory, and for
another, idlc is doomed to go away anyway.

Change-Id: I36f54130c8bfd1933126ae7f8a982e50d9bc616e