Add an experimental flag to Turbine to indicate to annotation processors that they are running in hjar compilation

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PiperOrigin-RevId: 145413255
MOS_MIGRATED_REVID=145413255
diff --git a/src/java_tools/buildjar/java/com/google/devtools/build/java/turbine/javac/JavacTurbine.java b/src/java_tools/buildjar/java/com/google/devtools/build/java/turbine/javac/JavacTurbine.java
index 85f4eaf..1152e3c 100644
--- a/src/java_tools/buildjar/java/com/google/devtools/build/java/turbine/javac/JavacTurbine.java
+++ b/src/java_tools/buildjar/java/com/google/devtools/build/java/turbine/javac/JavacTurbine.java
@@ -154,6 +154,9 @@
       argbuilder.add("-processor");
       argbuilder.add(Joiner.on(',').join(turbineOptions.processors()));
       processorpath = asPaths(turbineOptions.processorPath());
+
+      // see b/31371210
+      argbuilder.add("-Aexperimental_turbine_hjar");
     } else {
       processorpath = ImmutableList.of();
     }