swift234

Remove unused deserialized SILFunctions.

The deserializer holds a reference to the deserialized SILFunction, which
prevents Dead Function Elimination from erasing them. 

We have a tradeoff on how often we should clean up the unused deserialized
SILFunctions. If we clean up at every optimization iteration, we may
end up deserializing the same SILFunction multiple times. For now, we clean
up only after we are done with the optimization iteration.

rdar://17046033


Swift SVN r18697