libstdc++: Add file-io-diff to replace @diff@ markup in I/O tests

libstdc++: Add file-io-diff to replace @diff@ markup in I/O tests

This adds a new dg-final action to compare two files after a test has
run, so that we can verify that fstream operations produce the expected
results. With this change, all uses of @diff@ that seem potentially
useful have been converted to actually compare the files and FAIL if
they differ.

The file-io-diff action can take two arguments naming the files to be
compared, or for convenience it can take a single string and will
compare STR.tst and STR.txt, as that's how it's commonly used.

Additionally, all remaining uses of @require@ are converted to
dg-additional-files directives, so that the TODO in libstdc++.exp can
be resolved.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_filebuf/close/char/1.cc: Remove
	@require@ and @diff@. Use dg-final file-io-diff action.
	* testsuite/27_io/basic_istream/extractors_other/char/2.cc:
	Likewise.
	* testsuite/27_io/basic_istream/extractors_other/wchar_t/2.cc:
	Likewise.
	* testsuite/27_io/basic_istream/get/char/2.cc: Likewise.
	* testsuite/27_io/basic_istream/get/wchar_t/2.cc: Likewise.
	* testsuite/27_io/basic_istream/ignore/char/3.cc: Likewise.
	* testsuite/27_io/basic_istream/ignore/wchar_t/3.cc: Likewise.
	* testsuite/27_io/basic_istream/peek/char/6414.cc: Likewise.
	* testsuite/27_io/basic_istream/peek/wchar_t/6414.cc: Likewise.
	* testsuite/27_io/basic_istream/seekg/char/fstream.cc: Likewise.
	* testsuite/27_io/basic_istream/seekg/wchar_t/fstream.cc:
	Likewise.
	* testsuite/27_io/basic_istream/tellg/char/fstream.cc: Likewise.
	* testsuite/27_io/basic_istream/tellg/wchar_t/fstream.cc:
	Likewise.
	* testsuite/27_io/basic_ofstream/open/char/1.cc: Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/char/1.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/wchar_t/1.cc:
	Likewise.
	* testsuite/27_io/ios_base/sync_with_stdio/1.cc: Likewise.
	* testsuite/27_io/basic_ostream/inserters_other/char/2.cc:
	Likewise. Check file positions.
	* testsuite/27_io/basic_ostream/inserters_other/wchar_t/2.cc:
	Likewise.
	* testsuite/lib/libstdc++.exp (file-io-diff): New proc.