[NFC] Lazily Construct ErrorExpr/ErrorType When Default Arg Request Fails
[NFC] Lazily Construct ErrorExpr/ErrorType When Default Arg Request Fails

Preserve debuggability and save a bit of memory by only constructing
these when the request fails instead of every time.

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