Cross-Import Overlays

[beccadax]

This PR adds cross-import overlays to Swift. Cross-import overlays allow a module to declare that, when it is imported in the same file as a second module, one or more additional modules should be imported as well. It can be used, for instance, to automatically import test helpers when your module is imported along with XCTest.

This implementation intentionally deviates from the pitch thread in certain ways:

• We use YAML, rather than JSON, to specify the cross-import information. YAML was selected because, unlike JSON, it allows comments. Their file extension is now .swiftoverlay.
• Rather than storing overlays in .../Foo.swiftmodule/Overlays, we store them in .../Foo.swiftcrossimport. This allows "thin" (i.e. non-platform-specific) Swift modules to declare cross-import overlays.
• Clang modules can now declare cross-import overlays by placing a Foo.swiftcrossimport directory next to the module.modulemap file that defines them.
• A mechanism has been added to declare platform-specific (rather, module-target-triple-specific) cross-import overlays.
• Some edge cases are handled differently from the pitch; for instance, we don't emit a diagnostic if both modules declare the overlay.

Other differences are either unintentional, incomplete, or undecided.

Fixes rdar://problem/59444602.

[beccadax]

Feature-complete. To do:

• Rebase
• Self-review
• Clean up version history
• CI testing
• Peer review

[beccadax]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 8fae0de76f17ba1525930f33a1b8b814a950a227

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 8fae0de76f17ba1525930f33a1b8b814a950a227

[beccadax]

@varungandhi-apple I'm going to merge this, but feel free to post additional comments on the last round of changes.

[varungandhi-apple]

LGTM 😄

[drodriguez]

Just a note: a couple of tests seem broken on Windows (started in https://ci-external.swift.org/job/oss-swift-windows-x86_64/2907/ and https://ci-external.swift.org/job/oss-swift-windows-x86_64-vs2019/103/). One of the failing tests seems to be simple error message differences in Windows. The other seems to be some path comparison that doesn't normalize paths.

/cc @compnerd

[devincoughlin]

We'll look into this.

[devincoughlin]

I have a PR up at #29962 to unblock windows testing.

[varungandhi-apple]

Related PR with the diagnostic fix: [just to get the GitHub cross-linking to show up] #29965