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.
By explicitly marking children in the `schema.yml` file, an internal
`getAChild` predicate is implemented, that is in turn used in `AstNode`
to implement `getParent`.
This is yet to be used in the control flow library to replace the
hand-rolled implementation.
A further, more complex step is to use the same information to fully
generate the core implementation of `PrintAst` (including the
accessor string). This will be done later.
The `parent` tests use the same swift code as the extractor tests, and
this is currently enforced by `sync-files.py`. Notice that `qltest.sh`
had to be modified to deal with multiple files, which was not working
yet.
