Although I don't plan to bring over new assertions wholesale
into the current qualification branch, it's entirely possible
that various minor changes in main will use the new assertions;
having this basic support in the release branch will simplify that.
(This is why I'm adding the includes as a separate pass from
rewriting the individual assertions)
-typecheck-module-from-interface doesn't emit textual output as
configured in frontend option. The real output is diagnostics but that
is not tracked by output computer which is using the option.
Teach `swift-frontend` that it doesn't actually emit any real output and
tell serialization not to emit swiftmodule.
Using a virutal output backend to capture all the outputs from
swift-frontend invocation. This allows redirecting and/or mirroring
compiler outputs to multiple location using different OutputBackend.
As an example usage for the virtual outputs, teach swift compiler to
check its output determinism by running the compiler invocation
twice and compare the hash of all its outputs.
Virtual output will be used to enable caching in the future.
Push the top level logic for writing out swiftmodules and associated files into the frontend library which has access to all the necessary dependencies.
