[Serialization] Start getTypeChecked() -> TypeDeserializer

Like I just did for DeclDeserializer. This one adds the new class and
moves most of getTypeChecked() into it, but doesn't yet split anything
up.

Author: jrose-apple

include/swift/Serialization/ModuleFile.h

@@ -49,6 +49,7 @@ class ModuleFile
     public LazyConformanceLoader {
   friend class SerializedASTFile;
   friend class DeclDeserializer;
+  friend class TypeDeserializer;
   friend class SILDeserializer;
   using Status = serialization::Status;
   using TypeID = serialization::TypeID;