Rationalise PCH headers and 17_intro/headers tests.

Rationalise PCH headers and 17_intro/headers tests.

	PR libstdc++/64857
	* doc/xml/manual/using.xml: Improve aggregate header documentation.
	* doc/html/manual/*: Regenerate.
	* include/precompiled/extc++.h: Include <bits/stdc++.h> for C++11
	and later and include more extension headers.
	* testsuite/17_intro/headers/c++1998/all_attributes.cc: Remove
	redundant header.
	* testsuite/17_intro/headers/c++200x/: Rename to c++2011.
	* testsuite/17_intro/headers/c++2014/all_attributes.cc: Remove
	redundant headers.
	* testsuite/17_intro/headers/c++2014/all_no_exceptions.cc: New.
	* testsuite/17_intro/headers/c++2014/all_no_rtti.cc: New.
	* testsuite/17_intro/headers/c++2014/all_pedantic_errors.cc: New.
	* testsuite/17_intro/headers/c++2014/operator_names.cc: New.
	* testsuite/17_intro/headers/c++2014/stdc++.cc: New.
	* testsuite/17_intro/headers/c++2014/stdc++_multiple_inclusion.cc:
	New.

From-SVN: r227684