Allow a Swift 5.5 compiler to process the new _Concurrency module.
Allow a Swift 5.5 compiler to process the new _Concurrency module.

Swift 5.5 does not implement support for structural types that are
Sendable, which breaks type checking of the newer _Concurrency module.
Work around the issue with an `#if` in the offending inline code.

Fixes rdar://82602353.

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