Python: Add magic to named_argument_transfer

This predicate was materialised as a _big_, _cached_ relation:

```
(169s) Tuple counts for PointsTo::InterProceduralPointsTo::named_argument_transfer#ffff#join_rhs/4@38ce07 after 53.4s:
25212     ~4%     {3} r1 = SCAN Function::Function::getArgByName_dispred#fff OUTPUT In.1, In.0 'arg1', In.2 'arg2'
159751200 ~0%     {4} r2 = JOIN r1 WITH Flow::CallNode::getArgByName_dispred#fff_102#join_rhs ON FIRST 1 OUTPUT Rhs.1 'arg0', Lhs.1 'arg1', Lhs.2 'arg2', Rhs.2 'arg3'
                  return r2
```

... However it's only used in a single place (where it is immediately
joined with the points-to relation to relate the caller and argument),
none of these joins were ever larger than 2000 tuples. This made it
pretty clear that we could gain something by pushing in that points-to
join as a bit of manual magic.

However, doing so didn't actually fix anything, since the join-orderer
then decided to join `func.getArgByName(name)` with
`call.getArgByName(name)` on `name` as the first thing (which caused a
join of the same size as above).

Unbinding didn't work, since `name` would then be an unbound `string`,
so instead I factored out relating the function, parameter, and name
thereof into its own predicate. (I could also have done this with the
call, but I would expect there to be more calls than function
definitions in general.)

Overall, this resulted in going from

```
(709s)
Definitions.ql-7:PointsTo::InterProceduralPointsTo::named_argument_transfer#ffff#join_rhs ......... 53.5s
Definitions.ql-7:Instances::InstanceObject::initializer_dispred#fbf ............................... 35.3s (456 evaluations with max 136ms in Instances::InstanceObject::initializer_dispred#fbf/3@i110#0508e8)
Definitions.ql-10:DefinitionTracking::jump_to_defn_attribute#fbf .................................. 27s (100 evaluations with max 12.8s in DefinitionTracking::jump_to_defn_attribute#fbf/3@i1#fc1f7x)
Definitions.ql-7:PointsTo::PointsToInternal::pointsTo#ffff ........................................ 16.1s (681 evaluations with max 2.5s in PointsTo::PointsToInternal::pointsTo#ffff/4@i4#0508eg)
Definitions.ql-7:Constants::ConstantObjectInternal::attribute#ffff ................................ 13.4s (505 evaluations with max 50ms in Constants::ConstantObjectInternal::attribute#ffff/4@i153#0508e5)
Definitions.ql-10:DefinitionTracking::assignment_jump_to_defn_attribute#fbf ....................... 12.4s (99 evaluations with max 11.8s in DefinitionTracking::assignment_jump_to_defn_attribute#fbf/3@i2#fc1f
7z)
...
```

to

```
(668s)
Definitions.ql-7:Instances::InstanceObject::initializer_dispred#fbf ................... 35.4s (456 evaluations with max 140ms in Instances::InstanceObject::initializer_dispred#fbf/3@i110#bf4328)
Definitions.ql-10:DefinitionTracking::jump_to_defn_attribute#fbf ...................... 27.4s (100 evaluations with max 13.3s in DefinitionTracking::jump_to_defn_attribute#fbf/3@i1#679d7x)
Definitions.ql-7:PointsTo::PointsToInternal::pointsTo#ffff ............................ 16.1s (681 evaluations with max 2.5s in PointsTo::PointsToInternal::pointsTo#ffff/4@i4#bf432g)
Definitions.ql-7:Constants::ConstantObjectInternal::attribute#ffff .................... 14.4s (505 evaluations with max 51ms in Constants::ConstantObjectInternal::attribute#ffff/4@i140#bf4325)
Definitions.ql-10:DefinitionTracking::assignment_jump_to_defn_attribute#fbf ........... 12.3s (99 evaluations with max 11.7s in DefinitionTracking::assignment_jump_to_defn_attribute#fbf/3@i2#679d
7z)
...
```

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

@@ -1195,16 +1195,22 @@ module InterProceduralPointsTo {
     ControlFlowNode argument, PointsToContext caller, ParameterDefinition param,
     PointsToContext callee
   ) {
+    PointsToInternal::pointsTo(argument, caller, _, _) and
     exists(CallNode call, Function func, int offset |
       callsite_calls_function(call, caller, func, callee, offset)
     |
       exists(string name |
         argument = call.getArgByName(name) and
-        param.getParameter() = func.getArgByName(name)
+        function_parameter_name(func, param, name)
       )
     )
   }
 
+  pragma[nomagic]
+  private predicate function_parameter_name(Function func, ParameterDefinition param, string name) {
+    param.getParameter() = func.getArgByName(name)
+  }
+
   /**
    * Holds if the `call` with context `caller` calls the function `scope` in context `callee`
    * and the offset from argument to parameter is `parameter_offset`