Remove previous hack for SR-5206

[itaiferber]

What's in this pull request?
Remove remaining references to SR-5206 hack by bringing the implementation inline with the Foundation overlay.

Applies swiftlang/swift#11315 to swift-corelibs-foundation. This will go into the swift-4.0-branch as well.

[itaiferber]

@swift-ci Please test

[alblue]

@swift-ci please test

[alblue]

Looks like these failures aren't related - @spevans it seems to be to do with the NSLock tests?

Test Case 'TestNSLock.test_lockWait' started at 2017-10-12 11:06:43.671
TestFoundation/TestNSLock.swift:57: error: TestNSLock.test_lockWait : XCTAssertTrue failed - 
TestFoundation/TestNSLock.swift:58: error: TestNSLock.test_lockWait : XCTAssertTrue failed - 
TestFoundation/TestNSLock.swift:59: error: TestNSLock.test_lockWait : XCTAssertTrue failed - 
TestFoundation/TestNSLock.swift:60: error: TestNSLock.test_lockWait : XCTAssertTrue failed - 
Test Case 'TestNSLock.test_lockWait' failed (0.0 seconds)

[spevans]

@alblue, yes I think there might be a bug in the lock test unrelated to this change. I think I can see the issue so I'll create a PR which should hopefully fix it.

[spevans]

@alblue hopefully #1261 should fix it. This PR looks unrelated it looks like a testing on a loaded CI server issue.

[alblue]

@swift-ci please smoke test

[alblue]

@swift-ci please smoke test

[itaiferber]

@swift-ci Please clean test

[alblue]

@swift-ci please test

[alblue]

@itaiferber according to @shahmishal the foundation build doesn't understand smoke tests or clean tests ... so I've kicked it off again with the please test :)

[itaiferber]

@alblue Ah, right — I never remember which project supports what. Thanks!

[alblue]

@itaiferber that's ok, I only found out about half an hour ago too :)

[itaiferber]

I just looked in the swift-4.0-branch but it looks like these changes are already in there. ¯\(ツ)/¯ Less work for me, I guess...