[SourceKit] Remove dependency of SwiftLang on Core

[briancroom]

What's in this pull request?

SourceKit's SwiftLang (SwiftLangSupport class) and Core (Context class) libraries currently depend on each other. At @jrose-apple's suggestion, I have broken this coupling by requiring SwiftLangSupport to be injected into Context.

See also: #3080

---

Before merging this pull request to apple/swift repository:

• Test pull request on Swift continuous integration.

Triggering Swift CI

The swift-ci is triggered by writing a comment on this PR addressed to the GitHub user @swift-ci. Different tests will run depending on the specific comment that you use. The currently available comments are:

Smoke Testing

Platform	Comment
All supported platforms	@swift-ci Please smoke test
All supported platforms	@swift-ci Please smoke test and merge
OS X platform	@swift-ci Please smoke test OS X platform
Linux platform	@swift-ci Please smoke test Linux platform

Validation Testing

Platform	Comment
All supported platforms	@swift-ci Please test
All supported platforms	@swift-ci Please test and merge
OS X platform	@swift-ci Please test OS X platform
OS X platform	@swift-ci Please benchmark
Linux platform	@swift-ci Please test Linux platform

Lint Testing

Language	Comment
Python	@swift-ci Please Python lint

Note: Only members of the Apple organization can trigger swift-ci.

[briancroom]

@swift-ci please test

[gribozavr]

@akyrtzi @benlangmuir @nkcsgexi Could you review, please?

[akyrtzi]

I'd suggest a simpler way to break the cycle, which would result in much fewer changes.

-Change the Context constructor to something like this:
Context(StringRef RuntimeLibPath, llvm::function_ref<std::unique_ptr<LangSupport>(Context &)> LangSupportFactoryFn)
Have the constructor call the factory function to get the LangSupport pointer.

-Introduce static std::unique_ptr<LangSupport> SwiftLangSupport::create(SourceKit::Context &SKCtx) function

• At the points where Context is constructed also pass the above function reference.

[briancroom]

@akyrtzi oh nice, yeah that sounds like a great solution! I'll update the PR a bit later accordingly.

[briancroom]

@swift-ci please test

[briancroom]

Ok, I've pushed a simplified version now based on @akyrtzi's suggestion. Note that I opted to keep the new Factory.h header, because the SwiftLang module had no public headers at all previously, and I wanted to avoid requiring clients to depend on the entire SwiftLangSupport.h header. Does that seem sensible?

[briancroom]

@swift-ci please test

[akyrtzi]

Headers should generally include the headers that they depend on, in this case it should get a #include <memory> (because of std::unique_ptr)

[akyrtzi]

LGTM, with only a couple of nitpicks, thank you!

[briancroom]

@swift-ci Please test and merge