Avoid deprecation warning with -Wsystem-headers

C++17 says to use std::uncaught_exceptions() here instead of
std::uncaught_exception() but since we only care whether the result is
non-zero (and we aren't planning to remove the deprecated version) we
can just keep using std::uncaught_exception() and suppress the warning.

	* include/std/ostream (basic_ostream::sentry::~sentry): Suppress
	deprecation warnings for using uncaught_exception().

From-SVN: r263593

libstdc++-v3/ChangeLog

 2018-08-16  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/ostream (basic_ostream::sentry::~sentry): Suppress
+	deprecation warnings for using uncaught_exception().
+
 	PR libstdc++/86447
 	* src/c++11/cow-stdexcept.cc [_GLIBCXX_FULLY_DYNAMIC_STRING]
 	(logic_error::logic_error(logic_error&&))

libstdc++-v3/include/std/ostream

@@ -444,6 +444,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       explicit
       sentry(basic_ostream<_CharT, _Traits>& __os);
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
       /**
        *  @brief  Possibly flushes the stream.
        *
@@ -461,6 +463,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      _M_os.setstate(ios_base::badbit);
 	  }
       }
+#pragma GCC diagnostic pop
 
       /**
        *  @brief  Quick status checking.