Make ActionTemplate implement the interface StarlarkValue.

When creating an Action provider in /analysis/ActionsProvider.java the function Dict.immutableCopyOf(map) is called, which checks that all values are starlark values.  If one of the values is a LTOBackendActionTemplate, the following error is thrown: InvalidStarlarkValueException: invalid Starlark value: class LtoBackendActionTemplate

PiperOrigin-RevId: 553498116
Change-Id: Iad7bfe5b68fd766fc90b688f4fa4c469becdf9f0

src/main/java/com/google/devtools/build/lib/actions/ActionTemplate.java

@@ -20,6 +20,8 @@
 import com.google.devtools.build.lib.actions.Artifact.TreeFileArtifact;
 import com.google.devtools.build.lib.analysis.platform.PlatformInfo;
 import javax.annotation.Nullable;
+import net.starlark.java.eval.Printer;
+import net.starlark.java.eval.StarlarkValue;
 
 /**
  * A placeholder action that, at execution time, expands into a list of {@link Action}s to be
@@ -54,7 +56,7 @@
  *       not have artifact or artifact path prefix conflicts.
  * </ol>
  */
-public interface ActionTemplate<T extends Action> extends ActionAnalysisMetadata {
+public interface ActionTemplate<T extends Action> extends ActionAnalysisMetadata, StarlarkValue {
   /**
    * Given a set of input TreeFileArtifacts resolved at execution time, returns a list of expanded
    * actions to be executed.
@@ -108,4 +110,14 @@ default ImmutableMap<String, String> getExecProperties() {
   default PlatformInfo getExecutionPlatform() {
     return null;
   }
+
+  @Override
+  default void repr(Printer printer) {
+    printer.append(prettyPrint());
+  }
+
+  @Override
+  default boolean isImmutable() {
+    return true;
+  }
 }