Add visitor pattern for transition factories.

Part of #22248.

PiperOrigin-RevId: 632193752
Change-Id: Ic41afbc4ec508ad08615a8861278d5593bb73849

src/main/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionFactory.java

@@ -100,4 +100,10 @@ public boolean isTool() {
   public boolean isSplit() {
     return transitionFactory1().isSplit() || transitionFactory2().isSplit();
   }
+
+  @Override
+  public void visit(Visitor<T> visitor) {
+    this.transitionFactory1().visit(visitor);
+    this.transitionFactory2().visit(visitor);
+  }
 }

src/main/java/com/google/devtools/build/lib/analysis/config/transitions/TransitionFactory.java

@@ -76,4 +76,14 @@ default boolean isTool() {
   default boolean isSplit() {
     return false;
   }
+
+  /** Visit this trnsition factory with the given visitor. */
+  default void visit(Visitor<T> visitor) {
+    visitor.visit(this);
+  }
+
+  /** Interface used to progressively visit transitions. */
+  interface Visitor<T extends TransitionFactory.Data> {
+    void visit(TransitionFactory<T> factory);
+  }
 }