C++: Fix a bad join.

Before:
```
[2026-09-04 12:36:27] Evaluated non-recursive predicate FlowSummaryImpl::Input2::hasKindAndEnclosingFunction/3#3e350d0c@603f9du6 in 374ms (size: 604170).
Evaluated relational algebra for predicate FlowSummaryImpl::Input2::hasKindAndEnclosingFunction/3#3e350d0c@603f9du6 with tuple counts:
          672310  ~0%    {2} r1 = JOIN `cached_ResolveFunction::isFunction/1#9226b83f` WITH DataFlowPrivate::TSourceCallable#54d42094 ON FIRST 1 OUTPUT Rhs.1, Lhs.0
        34670069  ~2%    {2}    | JOIN WITH `DataFlowUtil::Node.getEnclosingCallable/0#dispred#74002437_10#join_rhs` ON FIRST 1 OUTPUT Rhs.1, Lhs.1
          604170  ~0%    {3}    | JOIN WITH `DataFlowPrivate::ReturnNode.getKind/0#dispred#c7586c0b` ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.0
                         return r1
```

After:
```
[2026-09-04 16:46:20] Evaluated non-recursive predicate FlowSummaryImpl::Input2::hasKindAndEnclosingFunction/3#3e350d0c@1ded09ff in 1158ms (size: 604170).
Evaluated relational algebra for predicate FlowSummaryImpl::Input2::hasKindAndEnclosingFunction/3#3e350d0c@1ded09ff with tuple counts:
        614924  ~1%    {3} r1 = JOIN `DataFlowPrivate::ReturnNode.getKind/0#dispred#c7586c0b` WITH `DataFlowUtil::Node.getEnclosingCallable/0#dispred#74002437` ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.0
        604170  ~5%    {3}    | JOIN WITH DataFlowPrivate::TSourceCallable#54d42094_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Lhs.2
        604170  ~5%    {3}    | JOIN WITH `cached_ResolveFunction::isFunction/1#9226b83f` ON FIRST 1 OUTPUT Lhs.0, Lhs.1, Lhs.2
                       return r1
```