AST: Be more careful about calls to ErrorType::get()
AST: Be more careful about calls to ErrorType::get()

It's nice to be able to set a breakpoint on this method when
debugging the type checker, but recently we started unconditionally
calling it to get the circularity sentinel before kicking off a
request.

Instead, let's only call it when we're in the failure path.

stephentyrone/swift,CodaFi/swift,hooman/swift,harlanhaskins/swift,gregomni/swift,nathawes/swift,ahoppen/swift,airspeedswift/swift,tkremenek/swift,gribozavr/swift,jmgc/swift,hooman/swift,airspeedswift/swift,aschwaighofer/swift,nathawes/swift,airspeedswift/swift,xwu/swift,jckarter/swift,karwa/swift,gregomni/swift,jmgc/swift,aschwaighofer/swift,airspeedswift/swift,gribozavr/swift,apple/swift,gregomni/swift,benlangmuir/swift,karwa/swift,jckarter/swift,tkremenek/swift,allevato/swift,nathawes/swift,airspeedswift/swift,rudkx/swift,gregomni/swift,aschwaighofer/swift,glessard/swift,atrick/swift,xwu/swift,JGiola/swift,JGiola/swift,lorentey/swift,hooman/swift,lorentey/swift,nathawes/swift,stephentyrone/swift,hooman/swift,parkera/swift,allevato/swift,allevato/swift,jmgc/swift,apple/swift,gribozavr/swift,nathawes/swift,karwa/swift,ahoppen/swift,rudkx/swift,xwu/swift,aschwaighofer/swift,karwa/swift,jckarter/swift,stephentyrone/swift,roambotics/swift,glessard/swift,rudkx/swift,ahoppen/swift,CodaFi/swift,roambotics/swift,lorentey/swift,benlangmuir/swift,aschwaighofer/swift,harlanhaskins/swift,roambotics/swift,lorentey/swift,harlanhaskins/swift,aschwaighofer/swift,ahoppen/swift,karwa/swift,jmgc/swift,lorentey/swift,gribozavr/swift,nathawes/swift,JGiola/swift,jckarter/swift,jckarter/swift,glessard/swift,lorentey/swift,harlanhaskins/swift,atrick/swift,JGiola/swift,gregomni/swift,jckarter/swift,xwu/swift,gribozavr/swift,ahoppen/swift,roambotics/swift,benlangmuir/swift,harlanhaskins/swift,apple/swift,hooman/swift,parkera/swift,harlanhaskins/swift,gregomni/swift,airspeedswift/swift,apple/swift,atrick/swift,atrick/swift,lorentey/swift,rudkx/swift,aschwaighofer/swift,parkera/swift,stephentyrone/swift,xwu/swift,CodaFi/swift,CodaFi/swift,parkera/swift,parkera/swift,allevato/swift,tkremenek/swift,benlangmuir/swift,jckarter/swift,CodaFi/swift,glessard/swift,airspeedswift/swift,roambotics/swift,karwa/swift,allevato/swift,rudkx/swift,apple/swift,gribozavr/swift,xwu/swift,jmgc/swift,harlanhaskins/swift,jmgc/swift,glessard/swift,stephentyrone/swift,gribozavr/swift,hooman/swift,tkremenek/swift,tkremenek/swift,atrick/swift,karwa/swift,allevato/swift,benlangmuir/swift,apple/swift,gribozavr/swift,glessard/swift,tkremenek/swift,atrick/swift,jmgc/swift,stephentyrone/swift,tkremenek/swift,stephentyrone/swift,CodaFi/swift,xwu/swift,allevato/swift,benlangmuir/swift,rudkx/swift,CodaFi/swift,JGiola/swift,parkera/swift,parkera/swift,nathawes/swift,roambotics/swift,parkera/swift,JGiola/swift,karwa/swift,lorentey/swift,ahoppen/swift,hooman/swift