Add objc_retainAutoreleasedReturnValue()

[spevans]

• This function has been duplicated in both swift-corelibs-libdispatch
  and swift-corelibs-foundation. Moving it to stdlib consolidates
  it and makes it available when not linking with Dispatch or
  Foundation.

• Only required for non-Apple targets as the Objc Runtime provides
  the appropiate function on Apple targets.

This needs to be tested/merged at the same time as swiftlang/swift-corelibs-libdispatch#258 which removes it from libdispatch otherwise there will be a duplicate symbol when testing libdispatch.

Foundation will be fixed in a followup PR but shouldn't have a problem with duplicate symbols as it currently links using --allow-multiple-definition

[gottesmm]

Are you sure that the right fix here isn't just to change the compiler?

[spevans]

@gottesmm That is probably the correct longterm fix and I was going to just alter it to emit a swift_retain instead of objc_retainAutoreleasedReturnValue however swift_retain doesn't return the original pointer. So I think any correct fix would require someone with more knowledge of the memory management.

Also I want to keep this change as simple as possible just to eliminate the duplicate symbols since it is a multi repo change, a better solution can be done in a followup PR.

[gottesmm]

Ok. Can you file an SR for that so we can track that work?

[gottesmm]

@swift-ci smoke test

[jckarter]

I don't think we should do this; this is unnecessary churn and just defining the symbol is hiding serious bugs. We should fix the compiler.

[spevans]

I opened https://bugs.swift.org/browse/SR-5271 to track the correct fix.