[noimplicitcopy] Refactor out a helper method and add some docs.
[noimplicitcopy] Refactor out a helper method and add some docs.

NFC.

JGiola/swift,glessard/swift,benlangmuir/swift,apple/swift,glessard/swift,benlangmuir/swift,roambotics/swift,apple/swift,roambotics/swift,roambotics/swift,apple/swift,glessard/swift,roambotics/swift,JGiola/swift,roambotics/swift,glessard/swift,JGiola/swift,benlangmuir/swift,apple/swift,JGiola/swift,apple/swift,roambotics/swift,glessard/swift,benlangmuir/swift,JGiola/swift,benlangmuir/swift,glessard/swift,apple/swift,benlangmuir/swift,JGiola/swift