rename LanguageToString to to_string

Bug: none
Test: aidl_unittests
Change-Id: I1451c5e75c78c8bb4f0ad9bf7bc7b99ebcfeb8db
diff --git a/options.h b/options.h
index 1ae4525..363b639 100644
--- a/options.h
+++ b/options.h
@@ -149,8 +149,6 @@
 
   bool GenApiMapping() const { return task_ == Task::DUMP_MAPPINGS; }
 
-  static const string LanguageToString(Language language);
-
   DiagnosticMapping GetDiagnosticMapping() const { return warning_options_.GetDiagnosticMapping(); }
 
   // The following are for testability, but cannot be influenced on the command line.
@@ -187,5 +185,7 @@
   WarningOptions warning_options_;
 };
 
+std::string to_string(Options::Language language);
+
 }  // namespace aidl
 }  // namespace android