Refactor cache writing out from internalCompile().

Add public writeCache() interface in bcc::Script.

Change-Id: If7b8ce56f7197e379613976c71c568ca0c019876
diff --git a/lib/ExecutionEngine/Script.h b/lib/ExecutionEngine/Script.h
index 870d354..73a2ce1 100644
--- a/lib/ExecutionEngine/Script.h
+++ b/lib/ExecutionEngine/Script.h
@@ -165,6 +165,7 @@
     int prepareExecutable(char const *cacheDir,
                           char const *cacheName,
                           unsigned long flags);
+    int writeCache();
 
     /*
      * Link the given bitcodes in mSourceList to shared object (.so).