[libcxx] Fix use of operator comma where the types can be user defined

Summary:
An evil user might overload operator comma. Use a void cast to make sure any user overload is not selected.
Modify all the test iterators to define operator comma. 

Reviewers: danalbert, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5929

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@220706 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/test_iterators.h

@@ -38,6 +38,9 @@ public:
     output_iterator& operator++() {++it_; return *this;}
     output_iterator operator++(int)
         {output_iterator tmp(*this); ++(*this); return tmp;}
+
+    template <class T>
+    void operator,(T const &);
 };
 
 template <class It>
@@ -71,6 +74,9 @@ public:
         {return x.it_ == y.it_;}
     friend bool operator!=(const input_iterator& x, const input_iterator& y)
         {return !(x == y);}
+
+    template <class T>
+    void operator,(T const &);
 };
 
 template <class T, class U>
@@ -120,6 +126,9 @@ public:
         {return x.it_ == y.it_;}
     friend bool operator!=(const forward_iterator& x, const forward_iterator& y)
         {return !(x == y);}
+
+    template <class T>
+    void operator,(T const &);
 };
 
 template <class T, class U>
@@ -168,6 +177,9 @@ public:
     bidirectional_iterator& operator--() {--it_; return *this;}
     bidirectional_iterator operator--(int)
         {bidirectional_iterator tmp(*this); --(*this); return tmp;}
+
+    template <class T>
+    void operator,(T const &);
 };
 
 template <class T, class U>
@@ -227,6 +239,9 @@ public:
         {random_access_iterator tmp(*this); tmp -= n; return tmp;}
 
     reference operator[](difference_type n) const {return it_[n];}
+
+    template <class T>
+    void operator,(T const &);
 };
 
 template <class T, class U>