[Localization] Simplify localization producer interface

Require producers to implement `getMessage(DiagID)` and
establish default implementation of `getMessageOr`, so
producers don't have to worry about handling fallback if
translation is unavailable.

Author: xedin

include/swift/Localization/LocalizationFormat.h

@@ -155,14 +155,20 @@ class SerializedLocalizationWriter {
 
 class LocalizationProducer {
 public:
-  /// If the  message isn't available/localized in the current `yaml` file,
+  /// If the  message isn't available/localized in current context
   /// return the fallback default message.
   virtual llvm::StringRef getMessageOr(swift::DiagID id,
                                        llvm::StringRef defaultMessage) const {
-    return defaultMessage;
+    auto message = getMessage(id);
+    return message.empty() ? defaultMessage : message;
   }
 
   virtual ~LocalizationProducer() {}
+
+protected:
+  /// Retrieve a message for the given diagnostic id.
+  /// \returns empty string if message couldn't be found.
+  virtual llvm::StringRef getMessage(swift::DiagID id) const = 0;
 };
 
 class YAMLLocalizationProducer final : public LocalizationProducer {
@@ -172,14 +178,15 @@ class YAMLLocalizationProducer final : public LocalizationProducer {
   /// The diagnostics IDs that are no longer available in `.def`
   std::vector<std::string> unknownIDs;
   explicit YAMLLocalizationProducer(llvm::StringRef filePath);
-  llvm::StringRef getMessageOr(swift::DiagID id,
-                               llvm::StringRef defaultMessage) const override;
 
   /// Iterate over all of the available (non-empty) translations
   /// maintained by this producer, callback gets each translation
   /// with its unique identifier.
   void forEachAvailable(
       llvm::function_ref<void(swift::DiagID, llvm::StringRef)> callback) const;
+
+protected:
+  llvm::StringRef getMessage(swift::DiagID id) const override;
 };
 
 class SerializedLocalizationProducer final : public LocalizationProducer {
@@ -193,8 +200,8 @@ class SerializedLocalizationProducer final : public LocalizationProducer {
   explicit SerializedLocalizationProducer(
       std::unique_ptr<llvm::MemoryBuffer> buffer);
 
-  llvm::StringRef getMessageOr(swift::DiagID id,
-                               llvm::StringRef defaultMessage) const override;
+protected:
+  llvm::StringRef getMessage(swift::DiagID id) const override;
 };
 
 class LocalizationInput : public llvm::yaml::Input {

lib/Localization/LocalizationFormat.cpp

@@ -95,15 +95,12 @@ SerializedLocalizationProducer::SerializedLocalizationProducer(
       base + tableOffset, base + sizeof(offset_type), base));
 }
 
-llvm::StringRef SerializedLocalizationProducer::getMessageOr(
-    swift::DiagID id, llvm::StringRef defaultMessage) const {
+llvm::StringRef
+SerializedLocalizationProducer::getMessage(swift::DiagID id) const {
   auto value = SerializedTable.get()->find(id);
-  llvm::StringRef diagnosticMessage((const char *)value.getDataPtr(),
-                                    value.getDataLen());
-  if (diagnosticMessage.empty())
-    return defaultMessage;
-
-  return diagnosticMessage;
+  if (value.getDataLen() == 0)
+    return llvm::StringRef();
+  return {(const char *)value.getDataPtr(), value.getDataLen()};
 }
 
 YAMLLocalizationProducer::YAMLLocalizationProducer(llvm::StringRef filePath) {
@@ -114,15 +111,8 @@ YAMLLocalizationProducer::YAMLLocalizationProducer(llvm::StringRef filePath) {
   unknownIDs = std::move(yin.unknownIDs);
 }
 
-llvm::StringRef
-YAMLLocalizationProducer::getMessageOr(swift::DiagID id,
-                                       llvm::StringRef defaultMessage) const {
-  if (diagnostics.empty())
-    return defaultMessage;
-  const std::string &diagnosticMessage = diagnostics[(unsigned)id];
-  if (diagnosticMessage.empty())
-    return defaultMessage;
-  return diagnosticMessage;
+llvm::StringRef YAMLLocalizationProducer::getMessage(swift::DiagID id) const {
+  return diagnostics[(unsigned)id];
 }
 
 void YAMLLocalizationProducer::forEachAvailable(