Change FunctionSummary.h's definition of SetOfDecls to be an ImmutableList instead

of a mutable SmallPtrSet.  While iterating over LocalTUDecls, there were cases
where we could modify LocalTUDecls, which could result in invalidating an iterator
and an analyzer crash.  Along the way, switch some uses of std::queue to std::dequeue,
which should be slightly more efficient.

Unfortunately, this is a difficult case to create a test case for.

llvm-svn: 155680