Constify (trivially) ImmutableSet::iterator::getVisitState().

This was probably intended all along.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156318 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/ImmutableSet.h

@@ -667,7 +667,7 @@ public:
     return reinterpret_cast<TreeTy*>(stack.back() & ~Flags);
   }
 
-  uintptr_t getVisitState() {
+  uintptr_t getVisitState() const {
     assert(!stack.empty());
     return stack.back() & Flags;
   }