Protect against evil overloaded operator comma in flat_[multi]set tests.

Author: vmichal

libcxx/test/std/containers/container.adaptors/flat.multiset/flat.multiset.iterators/iterator.pass.cpp

@@ -57,7 +57,7 @@ constexpr void test_one() {
   i                            = m.begin(); // move-assignment
   typename M::const_iterator k = i;         // converting constructor
   assert(i == k);                           // comparison
-  for (int j = 0; j < 9; ++j, ++i) {        // pre-increment
+  for (int j = 0; j < 9; ++j, (void)++i) {        // pre-increment
     assert(*i == expected[j]);              // operator*
   }
   assert(i == m.end());

libcxx/test/std/containers/container.adaptors/flat.set/flat.set.iterators/iterator.pass.cpp

@@ -56,7 +56,7 @@ constexpr void test_one() {
   i                            = m.begin(); // move-assignment
   typename M::const_iterator k = i;         // converting constructor
   assert(i == k);                           // comparison
-  for (int j = 1; j <= 4; ++j, ++i) {       // pre-increment
+  for (int j = 1; j <= 4; ++j, (void)++i) {       // pre-increment
     assert(*i == j);                        // operator*
   }
   assert(i == m.end());