swift234

[serialization] Initial -multi-file-output support.

If given a SourceFile, only decls within that SourceFile will be serialized.
Decls within other SourceFiles are emitted using cross-references that refer
to the current module.

Known issues:
- External definitions won't be serialized by any source file. They probably
  have to be serialized into /all/ of them.
- Nothing can actually /read/ a serialized partial-module yet. We need a
  notion of a TranslationUnit that can contain both source and serialized
  files, and we probably need loading to be more lazy.

Swift SVN r9978