Swift: _ as in _ = ... is a CFG leaf node.

This enables DataFlow to skip over it and not get stuck.

Author: d10c

swift/ql/lib/codeql/swift/controlflow/internal/ControlFlowGraphImpl.qll

@@ -1015,6 +1015,14 @@ module Decls {
 
 module Exprs {
   module AssignExprs {
+    /**
+     * The control-flow of a `DiscardAssignmentExpr`, which represents the
+     * `_` leaf expression that may appear on the left-hand side of an `AssignExpr`.
+     */
+    private class DiscardAssignmentExprTree extends AstLeafTree {
+      override DiscardAssignmentExpr ast;
+    }
+
     /**
      * The control-flow of an assignment operation.
      *