Intro the lit substitution
`%target-swift-emit-module-interfaces(Main.swiftinterface,
Main.private.swiftinterface` to generate both the public and private
swiftinterface. And use it in spi-only-import-exportability.swift.
To verify the generated interfaces I would recommend to keep two calls
to `%target-swift-typecheck-module-from-interface` as each is a
different invocation likely to fail.
