swift234

Move GameKit overlay to GameCenter.

In iOS 9 and OS X 10.11 the old GameKit was effectively renamed GameCenter, while
the new GameKit is a sort of umbrella framework like Cocoa. We need to support
backwards deployment, though, so the GameCenter overlay links to GameKit.framework.
(This is essentially the same solution implemented for CoreImage moving out of
QuartzCore in r28449)

rdar://problem/21340738

Swift SVN r30322