Add @Hide support to *.aidl

Test: runtests.sh ("aidl_unittests" only)
Test: Manually added @Hide to IShortcutService.aidl and checked the
    generated java file.
Bug: 150815464
Change-Id: Ie6362ce70ab7e1aefe6f443dd830cc75801897aa
diff --git a/aidl_language.h b/aidl_language.h
index 23638b0..7a40427 100644
--- a/aidl_language.h
+++ b/aidl_language.h
@@ -243,6 +243,7 @@
   bool IsUtf8InCpp() const;
   bool IsVintfStability() const;
   bool IsStableApiParcelable(Options::Language lang) const;
+  bool IsHide() const;
 
   void DumpAnnotations(CodeWriter* writer) const;