[Refactoring] Merge `LegacyAlternativeBodyCreator` into `AsyncConverter`
[Refactoring] Merge `LegacyAlternativeBodyCreator` into `AsyncConverter`

This will later allow us to reuse parts of `LegacyAlternativeBodyCreator` from `AsyncConverter` when refactoring calls to an async alternative if they pass a variable as the completion handler.

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