Add test for non-concurrency async-main
Add test for non-concurrency async-main

This patch adds a quick test to verify that we emit a reasonable error
message when trying to use async-main without enabling concurrency or
parsing as stdlib instead of crashing.

Under both circumstances, the _Concurrency library won't be implicitly
imported.

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