[polly] Change std::sort to llvm::sort in response to r327219

[mgrang]

Summary

r327219 added wrappers to std::sort which randomly shuffle the container before sorting.
This will help in uncovering non-determinism caused due to undefined sorting
order of objects having the same key.

To make use of that infrastructure we need to invoke llvm::sort instead of std::sort.

[sebpop]

LGTM.

lib/Support/ISLTools.cpp

static void printSortedPolyhedra(isl::union_set USet, llvm::raw_ostream &OS,
   });
 
   if (BSets.empty()) {
     OS << "{\n}\n";
     return;
   }
 
   // Sort the polyhedra.
-  std::sort(BSets.begin(), BSets.end(), orderComparer);
+  llvm::sort(BSets.begin(), BSets.end(), orderComparer);
 
   // Print the polyhedra.
   bool First = true;
   for (const isl::basic_set &BSet : BSets) {
     std::string Str;
     if (IsMap)
       Str = isl::map(BSet.unwrap()).to_str();
     else