[NFC] Remove caching for "isOverlay" computation when emitting a trace.
[NFC] Remove caching for "isOverlay" computation when emitting a trace.

Based on measurements, it seemed to save about 0.2 ms - 0.4 ms for a module
with a large number of imports from the SDK. That's insignificant compared to
the compile time for a typical module, which usually takes somewhere between
a few seconds and a few minutes.

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