swift234

[serialization] Codify the bringup hack with a special note in the module file.

The bringup hack, again, is to just process the module source files as a
TranslationUnit if the module doesn't accurately represent the original
source. Currently this happens if the module is not empty, or if it imports
/anything/, since we don't actually serialize anything yet.

This also cleans up the decl/type serialization skeleton a bit.

Swift SVN r5238