[Pitch] always export entry points in .swiftinterface files

[Slava_Pestov]

@jrose

But at the moment, it is not a project goal for swiftinterface files to be consumed by other languages; the way to interoperate with other languages is C and C++.

Yeah, I think instead of attempting to extend swiftinterface files to make them easier to consume directly, a better long-term goal is Implementing parts of the Swift compiler in Swift: semantic analysis should eventually be implemented in the form of reusable libraries, like swift-syntax already is today.