[Concurrency] Provide an alternative implementation when "async { }" is unusable
[Concurrency] Provide an alternative implementation when "async { }" is unusable

There exist Swift builds that support `Sendable` but not the attributes
used on `async`. Make sure that have an implementation of
`_runTaskForBridgedAsyncMethod` that does something.

Fixes rdar://77637570.

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