Python: Fix bad join in declaredAttributeVar

Before:
```
Tuple counts for PointsTo::declaredAttributeVar#fbf/3@99d5aenq after 1.1s:
451054   ~7%     {2} r1 = SCAN variable OUTPUT In.0, In.2 'name'
1296149  ~0%     {2} r2 = JOIN r1 WITH Essa::EssaVariable::getSourceVariable_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'var', Lhs.1 'name'
12179900 ~4%     {3} r3 = JOIN r2 WITH Essa::EssaVariable::getAUse_dispred#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'name', Lhs.0 'var'
8028     ~2%     {3} r4 = JOIN r3 WITH Scope::Scope::getANormalExit_dispred#bf_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'name', Lhs.2 'var'
8028     ~2%     {3} r5 = JOIN r4 WITH Classes::PythonClassObjectInternal::getScope_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'cls', Lhs.1 'name', Lhs.2 'var'
                 return r5
```

After:
```
Tuple counts for PointsTo::declaredAttributeVar#fbf/3@cccf36hb after 4ms:
1450 ~0%     {2} r1 = SCAN Classes::PythonClassObjectInternal::getScope_dispred#ff OUTPUT In.1, In.0 'cls'
1450 ~7%     {2} r2 = JOIN r1 WITH Scope::Scope::getANormalExit_dispred#bf ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'cls'
8028 ~0%     {2} r3 = JOIN r2 WITH Essa::EssaVariable::getAUse_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'var', Lhs.1 'cls'
8028 ~0%     {3} r4 = JOIN r3 WITH Essa::EssaVariable::getSourceVariable_dispred#ff ON FIRST 1 OUTPUT Rhs.1, Lhs.1 'cls', Lhs.0 'var'
8028 ~2%     {3} r5 = JOIN r4 WITH variable ON FIRST 1 OUTPUT Lhs.1 'cls', Rhs.2 'name', Lhs.2 'var'
return r5
```

Author: tausbn

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

@@ -2129,7 +2129,7 @@ module Conditionals {
 /** INTERNAL: Do not use. */
 predicate declaredAttributeVar(PythonClassObjectInternal cls, string name, EssaVariable var) {
   name = var.getName() and
-  var.getAUse() = cls.getScope().getANormalExit()
+  pragma[only_bind_into](pragma[only_bind_into](var).getAUse()) = cls.getScope().getANormalExit()
 }
 
 cached