Do not import objc_direct constructors

[ellishg]

Unlike normal Objective-C functions, Swift does not directly call
Objective-C constructors because it creates a thunk to allocate the
object and call then constructor dynamically. This can be fixed, but for
now emit a compile-time error rather than a runtime error.

[ellishg]

@rjmccall @CodaFi @slavapestov Could someone take a look? We ran into the unhandled case when Swift tries to call an ObjC constuctor marked as objc_direct.

[gottesmm]

@beccadax not sure if you are the correct reviewer, but I thought you would know better than me.

[beccadax]

I have some small requests, but this is basically sound.

Incidentally, I recently extracted some nearby code from importAttributes() into a new importSwiftAttrAttributes() method. Your change should still be in importAttributes(), not the new method.

[ellishg]

ping @beccadax could you take another quick look? I believe I've made the changes you suggested

[lanza]

@swift-ci please test

[lanza]

This LGTM if nobody else has any concerns!

[CodaFi]

@ellishg I've sketched lifting this restriction, and also moving [direct] into SIL itself, here https://github.com/CodaFi/swift/tree/airline-railway

[manman-ren]

@ellishg I've sketched lifting this restriction, and also moving [direct] into SIL itself, here https://github.com/CodaFi/swift/tree/airline-railway

@CodaFi Hey will you have time to upload the patch for review? Or it still needs some more work?
This is the last piece left to fully support direct ObjC methods from Swift.

Thanks!
Manman