std::regex_replace behaviour (LWG DR 2213)

[Jonathan Wakely]

The LWG have decided that
http://cplusplus.github.io/LWG/lwg-active.html#2213 is a defect.

In our std::regex_replace we do not appear to update out in all places
that we should.

I think we want this:

--- a/libstdc++-v3/include/bits/regex.tcc
+++ b/libstdc++-v3/include/bits/regex.tcc
@@ -512,7 +512,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       if (__i == __end)
        {
          if (!(__flags & regex_constants::format_no_copy))
-           std::copy(__first, __last, __out);
+           __out = std::copy(__first, __last, __out);
        }
       else
        {
@@ -528,7 +528,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
                break;
            }
          if (!(__flags & regex_constants::format_no_copy))
-           std::copy(__last.first, __last.second, __out);
+           __out = std::copy(__last.first, __last.second, __out);
        }
       return __out;
     }