[analyzer] Improve SymbolicRegion::dump() for heap pointers.

[NoQ]

Summary

Because for every symbol $x we can construct exactly two different MemRegions that both get dumped as SymRegion{$x}, and i didn't bother fixing it for years, i wanted to at least make the problem apparent. With this patch, the unknown-memory-spaced symbolic region remains SymRegion{$x}, while the heap-based memory region is dumped as HeapSymRegion{$x}.

Does anybody want me to add tests for this debug print?

[dcoughlin]

LGTM. Ship it!

lib/StaticAnalyzer/Core/MemRegion.cpp

 }
 
 void ObjCStringRegion::dumpToStream(raw_ostream &os) const {
   assert(Str != nullptr && "Expecting non-null ObjCStringLiteral");
   Str->printPretty(os, nullptr, PrintingPolicy(getContext().getLangOpts()));
 }
 
 void SymbolicRegion::dumpToStream(raw_ostream &os) const {
+  if (isa<HeapSpaceRegion>(getSuperRegion()))
+    os << "Heap";
   os << "SymRegion{" << sym << '}';
 }
 
 void VarRegion::dumpToStream(raw_ostream &os) const {
   os << *cast<VarDecl>(D);
 }
 
 LLVM_DUMP_METHOD void RegionRawOffset::dump() const {