[Typechecker] Allow @objc functions to return dynamic self

[theblixguy]

This PR lifts a restriction on @objc functions which disallowed dynamic self as return type.

@objc protocol Foo {
    static func makeFoo() -> Self // ok
}

Resolves SR-7601.

[theblixguy]

cc @jrose-apple

I am not sure if I should add a protocol restriction as well or just freely allow dynamic self as return type.

[jrose-apple]

This seems correct to me, but it's worth adding a PrintAsObjC test too to make sure it gets printed as instancetype. @slavapestov, any additional comments?

[slavapestov]

This change makes sense to me since we already import instancetype as Self. I would want to make sure PrintAsObjC can handle this first though.

[theblixguy]

@jrose-apple @slavapestov I have added a test file, does it look good to you?

[jrose-apple]

Great, looks like everything works. :-)

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - e65014e

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - e65014e

[theblixguy]

Whoops, there's a crash when running the new test file: If you're seeing a crash here, check that your SDK and dependencies are at least as new as the versions used to build 'dynamicself'

@jrose-apple do you know what could be the issue here?

[jrose-apple]

Ah, sorry. %clang-importer-sdk doesn't work with anything past SILGen; you'll have to copy over the "-enable-source-import hackaround" lines from one of the other PrintAsObjC tests. Or just move this into existing tests (the most boring ones being classes.swift and protocols.swift).

Sorry for not catching that in the review.

[theblixguy]

@jrose-apple oh, it's alright! I have updated the test file. Can you take a look and invoke the CI again if it looks good to you?

[jrose-apple]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 34c46d9

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 34c46d9

[theblixguy]

All tests have passed 🎉 @jrose-apple

[jrose-apple]

Thank you for the implementation!