libstdc++: Add deprecated attributes to old iostream members

libstdc++: Add deprecated attributes to old iostream members

Back in 2017 I removed these prehistoric members (which were deprecated
since C++98) for C++17 mode. But I didn't add deprecated attributes to
most of them, so users didn't get any warning they would be going away.
Apparently some poor souls do actually use some of these names, and so
now that GCC 11 defaults to -std=gnu++17 some code has stopped
compiling.

This adds deprecated attributes to them, so that C++98/03/11/14 code
will get a warning if it uses them. I'll also backport this to the
release branches so that users can find out about the deprecation before
they start using C++17.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_DEPRECATED_SUGGEST): New
	macro for "use 'foo' instead" message in deprecated warnings.
	* include/bits/ios_base.h (io_state, open_mode, seek_dir)
	(streampos, streamoff): Use _GLIBCXX_DEPRECATED_SUGGEST.
	* include/std/streambuf (stossc): Replace C++11 attribute
	with _GLIBCXX_DEPRECATED_SUGGEST.
	* testsuite/27_io/types/1.cc: Check for deprecated warnings.
	Also check for io_state, open_mode and seek_dir typedefs.

(cherry picked from commit eef9bf4ca8d90a1751bc4bff03722ee68999eb8e)