Python: Fix another bad "value transfer" join

The culprit:

```
Tuple counts for PointsTo::InterProceduralPointsTo::scope_entry_value_transfer_from_earlier#741b54e2#ffff#join_rhs/5@eb1340iv after 12.6s:
72973    ~3%     {2} r1 = JOIN PointsToContext::TImportContext#cf3039a0#f WITH Definitions::NonEscapingGlobalVariable#class#486534ab#f CARTESIAN PRODUCT OUTPUT Rhs.0, Lhs.0 'arg1'
537932   ~0%     {3} r2 = JOIN r1 WITH Essa::EssaDefinition::getSourceVariable#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'arg2', Lhs.1 'arg1', Lhs.0
982333   ~0%     {4} r3 = JOIN r2 WITH Essa::EssaVariable::getAUse#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.2, Lhs.1 'arg1', Lhs.0 'arg2', Rhs.1 'arg0'
37029774 ~0%     {4} r4 = JOIN r3 WITH Essa::TEssaNodeDefinition#24e22a14#ffff ON FIRST 1 OUTPUT Rhs.3 'arg3', Lhs.1 'arg1', Lhs.2 'arg2', Lhs.3 'arg0'
35956211 ~0%     {5} r5 = JOIN r4 WITH Essa::ScopeEntryDefinition::getScope#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.3 'arg0', Lhs.1 'arg1', Lhs.2 'arg2', Lhs.0 'arg3', Rhs.1 'arg4'
                return r5
```

You may notice that this is a predicate that's _materialised_, but it's
never actually used anywhere. It's the old "standard order" bringing
much sadness.

The problem here is that in the standard order (which we never actually
use here), we end up with a join between the bits above, `getRootCall`,
and `appliesToScope`. The `join_rhs` bit is joined twice, once with
`getRootCall#prev` and `appliesToScope#prev_delta` (in that order), and
once with `prev` and `prev_delta` swapped.

So to fix this, I used the unbinding pragma to force `appliesToScope` to
appear first in the join order. This was enough to make the compiler
_not_ push the common context into its own `join_rhs` predicate (and
the join-order is still decent.)

Author: tausbn

python/ql/lib/semmle/python/pointsto/PointsTo.qll

@@ -1271,12 +1271,21 @@ module InterProceduralPointsTo {
       )
     )
     or
+    non_escaping_global_transfer(pred_var, pred_context, succ_def, succ_context)
+  }
+
+  pragma[nomagic]
+  private predicate non_escaping_global_transfer(
+    EssaVariable pred_var, PointsToContext pred_context, ScopeEntryDefinition succ_def,
+    PointsToContext succ_context
+  ) {
     exists(NonEscapingGlobalVariable var |
       var = pred_var.getSourceVariable() and
       var = succ_def.getSourceVariable() and
       pred_var.getAUse() = succ_context.getRootCall() and
       pred_context.isImport() and
-      succ_context.appliesToScope(succ_def.getScope())
+      pragma[only_bind_into](succ_context)
+          .appliesToScope(pragma[only_bind_into](succ_def).getScope())
     )
   }