We already have something called "module interfaces" -- it's the
generated interface view that you can see in Xcode, the interface
that's meant for developers using a library. Of course, that's also a
textual format. To reduce confusion, rename the new module stability
feature to "parseable [module] interfaces".
The presence of a deinitializer will eventually indicate whether a
class's deinitializer is non-trivial for non-resilient modules.
Also improve recovery for normal source files for various bad ways
of declaring a deinitializer.
