This fixes `IfConfigDecl` for both the AST printer and control flow
libraries.
It turns out that the active lements of an `IfConfigDecl` are already
listed in the enclosing scope (like a `BraceStmt`), so they should not
be listed as children, and `IfConfigDecl` can be jsut a leaf in the
control flow.
The `ParentChild` tests have been generalized to test all `PrintAst`
by factoring out `PrintAstNode` into a separate file.
The `child.ql` and `parent.ql` tests have been removed as they are
subsumed by `PrintAst.ql`. Also, a new `no_parent_child_loops` is
added to detect back edges to a root node (back edges to a non-root
node are already detected by `no_double_parents.ql`).
