This will add helpers to get the underlying raw entities or constructor
arguments on stubs for synthesized classes.
For example a schema like:
```
@synth.from_class(A)
class B:
pass
@synth.on_arguments(base=A, index=int)
class C:
pass
```
will generate
```
cached
private Raw::A getUnderlyingEntity() { this = Synth::TB(result) }
```
in the `B.qll` stub and
```
cached
private Raw::A getUnderlyingBase() { this = Synth::TC(result, _) }
cached
private int getUnderlyingIndex() { this = Synth::TC(_, result) }
```
in the `C.qll` stub.
As stubs these can be freely changed later on.
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
