swift234

[serialization] Hook up basic name lookup for imported modules.

Also, explicitly list the top-level decls in a module. Eventually this
will be a proper lazily-loaded identifier-DeclID map, but for now it's
just a flat list of IDs to deserialize as soon as a lookup is
requested.

We can now parse and typecheck a file that imports typealiases of builtin
types.

Swift SVN r5325