Avoid deprecation warning with -Wsystem-headers

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