There was an issue in case multiple inheritance from classes with
children was involved, where indexes would overlap.
The generated code structure has been reshuffled a bit, with
`Impl::getImmediateChildOf<Class>` predicates giving 0-based children
for a given class, including those coming from bases, and the final
`Impl::getImmediateChild` disjuncting the above on final classes only.
This removes the need of `getMaximumChildrenIndex<Class>`, and also
removes the code scanning alerts.
Also, comments were fixed addressing the review.
Properties marked with `predicate` in the schema are now accepted.
* in the dbscheme, they will translate to a table with a single `id`
column (and the table name will not be pluralized)
* in C++ classes, they will translate to `bool` fields
* in QL classes, they will translate to predicates
Closes https://github.com/github/codeql-c-team/issues/1016
