[ParseableInterface] Fixes and tests for Graydon's fallback work

include/swift/Serialization/SerializedModuleLoader.h

@@ -20,6 +20,8 @@
 namespace swift {
 class ModuleFile;
 
+/// Spceifies how to load modules when both a parseable interface and serialized
+/// AST are present, or whether to disallow one format or the other altogether.
 enum class ModuleLoadingMode {
   PreferParseable,
   PreferSerialized,
@@ -56,6 +58,18 @@ protected:
                   std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
                   llvm::SmallVectorImpl<char> &Scratch);
 
+  /// If the module loader subclass knows that all options have been tried for
+  /// loading an architecture-specific file out of a swiftmodule bundle, try
+  /// to list the architectures that \e are present.
+  ///
+  /// \returns true if an error diagnostic was emitted
+  virtual bool maybeDiagnoseArchitectureMismatch(SourceLoc sourceLocation,
+                                                 StringRef moduleName,
+                                                 StringRef archName,
+                                                 StringRef directoryPath) {
+    return false;
+  }
+
 public:
   virtual ~SerializedModuleLoaderBase();
   SerializedModuleLoaderBase(const SerializedModuleLoaderBase &) = delete;
@@ -114,6 +128,18 @@ class SerializedModuleLoader : public SerializedModuleLoaderBase {
     : SerializedModuleLoaderBase(ctx, tracker, loadMode)
   {}
 
+  std::error_code
+  openModuleFiles(StringRef DirName, StringRef ModuleFilename,
+                  StringRef ModuleDocFilename,
+                  std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
+                  std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
+                  llvm::SmallVectorImpl<char> &Scratch) override;
+
+  bool maybeDiagnoseArchitectureMismatch(SourceLoc sourceLocation,
+                                         StringRef moduleName,
+                                         StringRef archName,
+                                         StringRef directoryPath) override;
+
 public:
   virtual ~SerializedModuleLoader();