[Serialization] Pull decl deserialization out to a helper class

In preparation for splitting up one big switch into many small
methods. Well, medium-sized methods.

No functionality change.

include/swift/Serialization/ModuleFile.h

@@ -48,6 +48,7 @@ class ModuleFile
   : public LazyMemberLoader,
     public LazyConformanceLoader {
   friend class SerializedASTFile;
+  friend class ASTDeserializer;
   friend class SILDeserializer;
   using Status = serialization::Status;
   using TypeID = serialization::TypeID;
@@ -526,10 +527,6 @@ private:
   std::unique_ptr<SerializedDeclMembersTable>
   readDeclMembersTable(ArrayRef<uint64_t> fields, StringRef blobData);
 
-  /// Main logic of getDeclChecked.
-  llvm::Expected<Decl *>
-  getDeclCheckedImpl(serialization::DeclID DID);
-
   /// Reads the index block, which contains global tables.
   ///
   /// Returns false if there was an error.