rename LanguageToString to to_string

Bug: none
Test: aidl_unittests
Change-Id: I1451c5e75c78c8bb4f0ad9bf7bc7b99ebcfeb8db
diff --git a/options.cpp b/options.cpp
index 58aea82..620edf6 100644
--- a/options.cpp
+++ b/options.cpp
@@ -155,7 +155,7 @@
   return sstr.str();
 }
 
-const string Options::LanguageToString(Language language) {
+string to_string(Options::Language language) {
   switch (language) {
     case Options::Language::CPP:
       return "cpp";