Fix interpolated string effect placement
Fix interpolated string effect placement

This fixes the placement of the await fix-it inside of interpolated
strings. We were putting the effect between the `\` and `(`, which
doesn't exactly work out very well. I'm intentionally being relatively
gentle with the casting when grabbing the body of the interpolated
string.

I can't think of a valid expression that doesn't result in that
structure, so it's probably safe to do a straight cast, and probably
should switch it over eventually. In the current state-of-affairs, it
will put the fix-it placement in the wrong place if my assumptions are
broken about the structure of the code.

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