[serialization] Remove FALL_BACK_TO_TRANSLATION_UNIT hack.

As a bring-up hack, the module serializer would write a special record,
FALL_BACK_TO_TRANSLATION_UNIT, if it encountered something it didn't know
how to serialize. This then directed the deserializer to ignore the
contents of the module file and instead reload the original source file.
Now that we can serialize pretty much everything*, though, we don't need
this, and instead we'd rather know where the serialization coverage has
gaps (by asserting).

Swift SVN r7752

lib/Serialization/ModuleFile.cpp

@@ -408,11 +408,6 @@ ModuleFile::ModuleFile(llvm::OwningPtr<llvm::MemoryBuffer> &&input)
       break;
     }
 
-    case FALL_BACK_TO_TRANSLATION_UNIT_ID:
-      // This is a bring-up hack and will eventually go away.
-      Status = ModuleStatus::FallBackToTranslationUnit;
-      break;
-
     default:
       // Unknown top-level block, possibly for use by a future version of the
       // module format.