swift234

[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