Fix a reentrance bug with deserializing ObjC type parameters.

This is a longstanding bug that seems to have been hidden by
a combination of (1) the normal flow being to deserialize the
interface before deserializing its parameter and (2) a precise
ordering of work that was apparently recently disturbed,
perhaps by my abstract-serialization work or Bruno's ObjC
module merging work.

Fixes rdar://59153545.