Python: Add getMaDRepresentation()

This adds a convenient way of getting the Models-as-Data representation
of a particular type of content. This avoids repeating the same
construction over and over in our various summaries. Currently this is
defined for all types of content except the captured variable content,
which to my knowledge doesn't have any representation in Models-as-Data.

python/ql/lib/semmle/python/dataflow/new/internal/DataFlowPublic.qll

@@ -687,16 +687,23 @@ newtype TContent =
 class Content extends TContent {
   /** Gets a textual representation of this element. */
   string toString() { result = "Content" }
+
+  /** Gets the Models-as-Data representation of this content (if any). */
+  string getMaDRepresentation() { none() }
 }
 
 /** An element of a list. */
 class ListElementContent extends TListElementContent, Content {
   override string toString() { result = "List element" }
+
+  override string getMaDRepresentation() { result = "ListElement" }
 }
 
 /** An element of a set. */
 class SetElementContent extends TSetElementContent, Content {
   override string toString() { result = "Set element" }
+
+  override string getMaDRepresentation() { result = "SetElement" }
 }
 
 /** An element of a tuple at a specific index. */
@@ -709,6 +716,8 @@ class TupleElementContent extends TTupleElementContent, Content {
   int getIndex() { result = index }
 
   override string toString() { result = "Tuple element at index " + index.toString() }
+
+  override string getMaDRepresentation() { result = "TupleElement[" + index + "]" }
 }
 
 /** An element of a dictionary under a specific key. */
@@ -721,11 +730,15 @@ class DictionaryElementContent extends TDictionaryElementContent, Content {
   string getKey() { result = key }
 
   override string toString() { result = "Dictionary element at key " + key }
+
+  override string getMaDRepresentation() { result = "DictionaryElement[" + key + "]" }
 }
 
 /** An element of a dictionary under any key. */
 class DictionaryElementAnyContent extends TDictionaryElementAnyContent, Content {
   override string toString() { result = "Any dictionary element" }
+
+  override string getMaDRepresentation() { result = "DictionaryElementAny" }
 }
 
 /** An object attribute. */
@@ -738,6 +751,8 @@ class AttributeContent extends TAttributeContent, Content {
   string getAttribute() { result = attr }
 
   override string toString() { result = "Attribute " + attr }
+
+  override string getMaDRepresentation() { result = "Attribute[" + attr + "]" }
 }
 
 /** A captured variable. */
@@ -750,6 +765,8 @@ class CapturedVariableContent extends Content, TCapturedVariableContent {
   VariableCapture::CapturedVariable getVariable() { result = v }
 
   override string toString() { result = "captured " + v }
+
+  override string getMaDRepresentation() { none() }
 }
 
 /**