Introduce no-op --experimental_google_legacy_api flag

This flag will be used to guard a substantial amount of experimental-only Starlark Build API pertaining to Google legacy code.

RELNOTES: None.
PiperOrigin-RevId: 244378067
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
index 5d78a88..fc5ab5d 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/StarlarkSemantics.java
@@ -120,6 +120,8 @@
 
   public abstract boolean experimentalEnableAndroidMigrationApis();
 
+  public abstract boolean experimentalGoogleLegacyApi();
+
   public abstract ImmutableList<String> experimentalJavaCommonCreateProviderEnabledPackages();
 
   public abstract boolean experimentalPlatformsApi();
@@ -208,6 +210,7 @@
           .experimentalBuildSettingApi(false)
           .experimentalCcSkylarkApiEnabledPackages(ImmutableList.of())
           .experimentalEnableAndroidMigrationApis(false)
+          .experimentalGoogleLegacyApi(false)
           .experimentalJavaCommonCreateProviderEnabledPackages(ImmutableList.of())
           .experimentalPlatformsApi(false)
           .experimentalRestrictNamedParams(false)
@@ -256,6 +259,8 @@
 
     public abstract Builder experimentalEnableAndroidMigrationApis(boolean value);
 
+    public abstract Builder experimentalGoogleLegacyApi(boolean value);
+
     public abstract Builder experimentalJavaCommonCreateProviderEnabledPackages(List<String> value);
 
     public abstract Builder experimentalPlatformsApi(boolean value);