[irgen] Implement irgen::IRBuilder::SavedInsertionPointRAII.
[irgen] Implement irgen::IRBuilder::SavedInsertionPointRAII.

This is ported from the SavedInsertionPointRAII at the SIL level. I thought that
I needed this to accomplish some of my recent work, but I was wrong. Still
seemed useful enough to upstream it.

rudkx/swift,JGiola/swift,apple/swift,atrick/swift,ahoppen/swift,ahoppen/swift,gregomni/swift,rudkx/swift,gregomni/swift,benlangmuir/swift,apple/swift,roambotics/swift,glessard/swift,JGiola/swift,roambotics/swift,apple/swift,benlangmuir/swift,ahoppen/swift,atrick/swift,atrick/swift,gregomni/swift,gregomni/swift,glessard/swift,JGiola/swift,apple/swift,ahoppen/swift,apple/swift,apple/swift,glessard/swift,rudkx/swift,glessard/swift,ahoppen/swift,benlangmuir/swift,benlangmuir/swift,JGiola/swift,atrick/swift,rudkx/swift,atrick/swift,atrick/swift,roambotics/swift,benlangmuir/swift,gregomni/swift,roambotics/swift,ahoppen/swift,glessard/swift,roambotics/swift,gregomni/swift,glessard/swift,rudkx/swift,JGiola/swift,benlangmuir/swift,roambotics/swift,rudkx/swift,JGiola/swift