swift234

[serialization] Sketch out high-level decl/type serialization.

This is basically modeled off of Clang's ASTWriter: write out all decls
and types en masse, then write out a list of offsets for quick access later,
and use the indexes into the offset lists as module-unique IDs for the decls
and types.

No decls or types are actually being serialized yet, and I haven't done
any of the work necessary for multi-module support (which would assume
offsets don't start at 0). There's also no reader support yet.

Swift SVN r5237