Change QMS iterator to take const QMS* in constructor.

Summary: The functions in QMS are all marked as const already.

Reviewed By: philippv

Differential Revision: D18240371

fbshipit-source-id: 113f5c968207f0a4ff9b780cd43de43cb90f758b

folly/experimental/QuotientMultiSet-inl.h

@@ -288,7 +288,7 @@ uint64_t QuotientMultiSet<Instructions>::getBlockPayload(
 
 template <class Instructions>
 QuotientMultiSet<Instructions>::Iterator::Iterator(
-    QuotientMultiSet<Instructions>* qms)
+    const QuotientMultiSet<Instructions>* qms)
     : qms_(qms),
       key_(0),
       occBlockIndex_(-1),

folly/experimental/QuotientMultiSet.h

@@ -176,7 +176,7 @@ class QuotientMultiSet final {
 template <class Instructions>
 class QuotientMultiSet<Instructions>::Iterator {
  public:
-  explicit Iterator(QuotientMultiSet<Instructions>* qms);
+  explicit Iterator(const QuotientMultiSet<Instructions>* qms);
 
   // Advance to the next key.
   bool next();
@@ -209,7 +209,7 @@ class QuotientMultiSet<Instructions>::Iterator {
   // Move to next occupied.
   bool nextOccupied();
 
-  QuotientMultiSet<Instructions>* qms_;
+  const QuotientMultiSet<Instructions>* qms_;
   uint64_t key_;
 
   // State members for the quotient occupied position.