CompoundVal now uses an ImmutableList<SVal> to store its set of SVals.  This change was motivated by the need to allow state-splitting in GRExprEngine::VisitInitListExpr.  As a side-benefit, we no longer need to perform any copies of SVals when creating a CompoundSVal, and the profiling of CompoundSVal is now constant time.

llvm-svn: 58437