swift234

[Serialization] If the output swiftmodule hasn't changed, don't touch the previous one.

Compiler output at least up to serialization should be deterministic at this point,
at least when not taking SIL into account. This /should/ mean that changing a
function body should not affect the final built swiftmodule, which means downstream
targets don't need to be rebuilt. Leaving the previous swiftmodule output in place
signals that.

A while back I put in a push to get all the non-determinism out of type checking,
importing, and serialization itself; it looks like we've finally made it. Let's keep
it that way!

rdar://problem/20539158 and others

Swift SVN r29923