swift234

[serialization] Add cross-module decl references.

This is currently implemented in terms of an access path of identifiers,
with a type check at the very end to pick the correct overload for a
function. Of course there are plenty of things missing here:
- function overloading on parameter names (selector pieces)
- access to values inside extensions
- generics (as everywhere)

This allows the serialization tests to be built against the standard
library instead of requiring -parse-stdlib.

Swift SVN r5797