Shared CFG: add defaulted getWhileElse/getForeachElse to AstSig

Lifts the shared-controlflow changes out of the larger Python new-CFG
PR so they can be reviewed independently. No behavioural change for
existing languages (Java, C#, Ruby, Swift, Go, ...) — both new
predicates default to `none()` and the Make0 loop-edge case
extensions only fire when a language overrides them.

Python's upcoming AstNodeImpl wiring uses these predicates to model
`while-else` and `for-else`, where the `else` block runs when the
loop condition becomes false (rather than via a `break`).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

shared/controlflow/change-notes/2026-05-19-loop-else.md

@@ -0,0 +1,4 @@
+---
+category: feature
+---
+* The `AstSig` signature gains two new defaulted predicates `getWhileElse` and `getForeachElse`, allowing languages (like Python) to model `while-else` / `for-else` constructs where the `else` branch is taken when the loop condition becomes false (rather than via a `break`). Existing languages that do not provide these predicates retain the previous behaviour.

shared/controlflow/codeql/controlflow/ControlFlowGraph.qll

@@ -211,6 +211,20 @@ signature module AstSig<LocationSig Location> {
    */
   default AstNode getTryElse(TryStmt try) { none() }
 
+  /**
+   * Gets the `else` block of this `while` loop statement, if any.
+   *
+   * Only some languages (e.g. Python) support `while-else` constructs.
+   */
+  default AstNode getWhileElse(WhileStmt loop) { none() }
+
+  /**
+   * Gets the `else` block of this `foreach` loop statement, if any.
+   *
+   * Only some languages (e.g. Python) support `for-else` constructs.
+   */
+  default AstNode getForeachElse(ForeachStmt loop) { none() }
+
   /** A catch clause in a try statement. */
   class CatchClause extends AstNode {
     /** Gets the variable declared by this catch clause. */
@@ -1549,19 +1563,32 @@ module Make0<LocationSig Location, AstSig<Location> Ast> {
           n2.isBefore(loopstmt.getBody())
           or
           n1.isAfterFalse(cond) and
-          n2.isAfter(loopstmt)
+          (
+            n2.isBefore(getWhileElse(loopstmt))
+            or
+            not exists(getWhileElse(loopstmt)) and n2.isAfter(loopstmt)
+          )
           or
           n1.isAfter(loopstmt.getBody()) and
           n2.isAdditional(loopstmt, loopHeaderTag())
         )
         or
+        exists(WhileStmt whilestmt |
+          n1.isAfter(getWhileElse(whilestmt)) and
+          n2.isAfter(whilestmt)
+        )
+        or
         exists(ForeachStmt foreachstmt |
           n1.isBefore(foreachstmt) and
           n2.isBefore(foreachstmt.getCollection())
           or
           n1.isAfterValue(foreachstmt.getCollection(),
             any(EmptinessSuccessor t | t.getValue() = true)) and
-          n2.isAfter(foreachstmt)
+          (
+            n2.isBefore(getForeachElse(foreachstmt))
+            or
+            not exists(getForeachElse(foreachstmt)) and n2.isAfter(foreachstmt)
+          )
           or
           n1.isAfterValue(foreachstmt.getCollection(),
             any(EmptinessSuccessor t | t.getValue() = false)) and
@@ -1574,10 +1601,17 @@ module Make0<LocationSig Location, AstSig<Location> Ast> {
           n2.isAdditional(foreachstmt, loopHeaderTag())
           or
           n1.isAdditional(foreachstmt, loopHeaderTag()) and
-          n2.isAfter(foreachstmt)
+          (
+            n2.isBefore(getForeachElse(foreachstmt))
+            or
+            not exists(getForeachElse(foreachstmt)) and n2.isAfter(foreachstmt)
+          )
           or
           n1.isAdditional(foreachstmt, loopHeaderTag()) and
           n2.isBefore(foreachstmt.getVariable())
+          or
+          n1.isAfter(getForeachElse(foreachstmt)) and
+          n2.isAfter(foreachstmt)
         )
         or
         exists(ForStmt forstmt, PreControlFlowNode condentry |