[NFC][libc++][format] Uses stringstream::view.

[Mordante]

Summary

This member has been added in D148641 so it can be used in the formatter
to avoid creating a "temporary" string.

libcxx/include/__chrono/formatter.h

if constexpr (chrono::__is_duration<_Tp>::value) {
         if (__value.is_negative())
           __sstr << _CharT('-');
       }
 
       __formatter::__format_chrono_using_chrono_specs(__value, __sstr, __chrono_specs);
     }
   }
 
-  // TODO FMT Use the stringstream's view after P0408R7 has been implemented.
-  basic_string<_CharT> __str = __sstr.str();
-  return __formatter::__write_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);
+  return __formatter::__write_string(__sstr.view(), __ctx.out(), __specs);
 }
 
 } // namespace __formatter
 
 template <__fmt_char_type _CharT>
 struct _LIBCPP_TEMPLATE_VIS __formatter_chrono {
 public:
   template <class _ParseContext>