[Serialization] Refactor subset of ModuleFile into ModuleFileSharedCore

The difference with `ModuleFile` is that `ModuleFileSharedCore` provides immutable data and is independent of a particular ASTContext.
It is designed to be able to be shared across multiple `ModuleFile`s of different `ASTContext`s in a thread-safe manner.

include/swift/Serialization/SerializedModuleLoader.h

@@ -58,7 +58,7 @@ class SerializedModuleLoaderBase : public ModuleLoader {
   using LoadedModulePair = std::pair<std::unique_ptr<ModuleFile>, unsigned>;
   std::vector<LoadedModulePair> LoadedModuleFiles;
 
-  SmallVector<std::unique_ptr<llvm::MemoryBuffer>, 2> OrphanedMemoryBuffers;
+  SmallVector<std::unique_ptr<ModuleFile>, 2> OrphanedModuleFiles;
 
 protected:
   ASTContext &Ctx;