[move-function] Implement SILFunction::isDefer().
[move-function] Implement SILFunction::isDefer().

Returns true if this SILFunction must be a defer.

NOTE: This may return false for defer statements that have been
deserialized without a DeclContext. This means that this is guaranteed to
be correct for SILFunctions in Raw SIL that were not deserialized as
canonical. Thus one can use it for diagnostics.

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