[Analyzer] Non-determinism: stable iteration on indirect goto LabelDecl's

CFG wass built in non-deterministic order due to the fact that indirect
goto labels' declarations (LabelDecl's) are stored in the llvm::SmallSet
container. LabelDecl's are pointers, whose order is not deterministic,
and llvm::SmallSet sorts them by their non-deterministic addresses after
"small" container is exceeded. This leads to non-deterministic processing
of the elements of the container.

The fix is to use llvm::SmallSetVector that was designed to have 
deterministic iteration order.

Patch by Ilya Palachev!

Differential Revision: https://reviews.llvm.org/D40073

llvm-svn: 318750