[CMake] Sema depends on Serialization these days.

[jrose-apple]

Specifically, it uses SerializedASTFile::getLanguageVersionBuiltWith to improve diagnostics. Not having this has led to failures on Linux, where linking order matters.

[jrose-apple]

@swift-ci Please smoke test

[jrose-apple]

Merging anyway to unblock my other commit, but I still need someone to sign off on it for the 4.0 branch. Joe, I tagged you just because IIRC you added the use.

[jckarter]

Is there a better refactoring possible that loosens the coupling? Maybe instead of what we have today:

          auto *superFile = Super->getModuleScopeContext();
          if (auto *serialized = dyn_cast<SerializedASTFile>(superFile)) {
            if (serialized->getLanguageVersionBuiltWith() !=
                TC.getLangOpts().EffectiveLanguageVersion) {

We could add Optional<Version> getLanguageVersionBuiltWith() as an API on DeclContext, so you don't need to cast to a specific serialization type?

[jrose-apple]

Yeah, looks like this isn't good enough anyway because we have a separate, unrelated circular dependency: Sema -> Parse -> SIL -> Sema. I think that's what's messing it up.

I'd move the API to FileUnit instead of SerializedASTFile if we were going to move it anywhere.