Revert "Oops, bad tracking of displacement on stride_counting_iterator"

This reverts commit 995d04b.

Author: ichaer

libcxx/test/support/test_iterators.h

@@ -826,7 +826,7 @@ class stride_counting_iterator {
         It tmp(base_);
         base_ = base(tmp += n);
         ++*stride_count_;
-        *stride_displacement_ += n;
+        ++*stride_displacement_;
         return *this;
     }
 
@@ -836,7 +836,7 @@ class stride_counting_iterator {
         It tmp(base_);
         base_ = base(tmp -= n);
         ++*stride_count_;
-        *stride_displacement_ -= n;
+        --*stride_displacement_;
         return *this;
     }