* include/std/std_fstream.h (basic_filebuf::sync): Hoist

	* include/std/std_fstream.h (basic_filebuf::sync): Hoist
	unconditional flush on lower-layer handle to here...
	* include/bits/fstream.tcc (basic_filebuf::_M_really_overflow):
	...from here.  Optimize remaining _M_file.sync() call pattern.
	* testsuite/27_io/narrow_stream_objects.cc (test04): New test.
	(test05): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@52699 138bc75d-0d04-0410-961f-82ee72b054a4