We need to serialize the underlying type substitution map for an
inlinable function. However, there is no reason to deserialize it
eagerly, since doing so can lead to cycles. It is better for
correctness and performance to only deserialize it when needed.
Technically this fixes a regression from #84299, but the actual
problem was there all along, it was just exposed by my change
on a specific project.
Fixes rdar://163301203.
27097430cc