Disable availability checking when type-checking the _Concurrency interface
Disable availability checking when type-checking the _Concurrency interface

Swift 5.5 didn't support back deployment of concurrency features, so a
Swift 5.5 compiler processing the _Concurrency .swiftinterface will
produce errors for each `async` function and actor with pre-macOS
12/iOS 15 availability. Emit `-disable-availability-checking` into the
generated `.swiftinterface` files to allow Swift 5.5 to continue to
build them.

Finishes rdar://82602353.

roambotics/swift,JGiola/swift,rudkx/swift,apple/swift,atrick/swift,roambotics/swift,rudkx/swift,ahoppen/swift,glessard/swift,benlangmuir/swift,roambotics/swift,roambotics/swift,JGiola/swift,xwu/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,apple/swift,atrick/swift,JGiola/swift,glessard/swift,xwu/swift,atrick/swift,xwu/swift,JGiola/swift,roambotics/swift,glessard/swift,gregomni/swift,xwu/swift,JGiola/swift,JGiola/swift,xwu/swift,gregomni/swift,glessard/swift,ahoppen/swift,rudkx/swift,atrick/swift,glessard/swift,rudkx/swift,xwu/swift,benlangmuir/swift,benlangmuir/swift,gregomni/swift,apple/swift,ahoppen/swift,apple/swift,apple/swift,roambotics/swift,atrick/swift,ahoppen/swift,gregomni/swift,gregomni/swift,gregomni/swift,rudkx/swift,xwu/swift,ahoppen/swift,rudkx/swift,benlangmuir/swift,atrick/swift,apple/swift