Add SerializeSIL.cpp for basic implementation of a SIL serializer.
serialize, serializeToStream, Serializer::writeToStream, and
Serializer::writeTranslationUnit all take an additional SILModule to pass it
to Serializer::writeSILFunctions.
Serializer::writeSILFunctions goes through all SILFunctions in a SILModule, and
serializes all SILFunctions that are transparent.
Swift SVN r7875
