rename LanguageToString to to_string

Bug: none
Test: aidl_unittests
Change-Id: I1451c5e75c78c8bb4f0ad9bf7bc7b99ebcfeb8db
diff --git a/aidl_language.cpp b/aidl_language.cpp
index 5389a4f..d7d5760 100644
--- a/aidl_language.cpp
+++ b/aidl_language.cpp
@@ -1083,8 +1083,7 @@
                                                    Options::Language lang) const {
   if ((lang == Options::Language::NDK || lang == Options::Language::RUST) && IsArray() &&
       GetName() == "IBinder") {
-    AIDL_ERROR(this) << "The " << Options::LanguageToString(lang)
-                     << " backend does not support array of IBinder";
+    AIDL_ERROR(this) << "The " << to_string(lang) << " backend does not support array of IBinder";
     return false;
   }
   if (lang == Options::Language::RUST && GetName() == "ParcelableHolder") {
@@ -1095,22 +1094,21 @@
   if ((lang == Options::Language::NDK || lang == Options::Language::RUST) && IsArray() &&
       IsNullable()) {
     if (GetName() == "ParcelFileDescriptor") {
-      AIDL_ERROR(this) << "The " << Options::LanguageToString(lang)
+      AIDL_ERROR(this) << "The " << to_string(lang)
                        << " backend does not support nullable array of ParcelFileDescriptor";
       return false;
     }
 
     const auto defined_type = typenames.TryGetDefinedType(GetName());
     if (defined_type != nullptr && defined_type->AsParcelable() != nullptr) {
-      AIDL_ERROR(this) << "The " << Options::LanguageToString(lang)
+      AIDL_ERROR(this) << "The " << to_string(lang)
                        << " backend does not support nullable array of parcelable";
       return false;
     }
   }
   if (this->GetName() == "FileDescriptor" &&
       (lang == Options::Language::NDK || lang == Options::Language::RUST)) {
-    AIDL_ERROR(this) << "FileDescriptor isn't supported by the " << Options::LanguageToString(lang)
-                     << " backend.";
+    AIDL_ERROR(this) << "FileDescriptor isn't supported by the " << to_string(lang) << " backend.";
     return false;
   }
   if (this->IsGeneric()) {