P0754R2 <version> header

P0754R2 <version> header

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/c++config: Change doxygen comment to suggest <version>
	instead of <iosfwd>.
	* include/precompiled/stdc++.h: Include <cwchar> and <cwctype>
	unconditionally.  Add C++17 and C++20 headers.
	* include/std/version: New header.
	* testsuite/17_intro/headers/c++2017/all_attributes.cc: New.
	* testsuite/17_intro/headers/c++2017/all_no_exceptions.cc: New.
	* testsuite/17_intro/headers/c++2017/all_no_rtti.cc: New.
	* testsuite/17_intro/headers/c++2017/all_pedantic_errors.cc: New.
	* testsuite/17_intro/headers/c++2017/operator_names.cc: New.
	* testsuite/17_intro/headers/c++2017/stdc++.cc: New.
	* testsuite/17_intro/headers/c++2017/stdc++_multiple_inclusion.cc:
	New.
	* testsuite/17_intro/headers/c++2020/all_attributes.cc: New.
	* testsuite/17_intro/headers/c++2020/all_no_exceptions.cc: New.
	* testsuite/17_intro/headers/c++2020/all_no_rtti.cc: New.
	* testsuite/17_intro/headers/c++2020/all_pedantic_errors.cc: New.
	* testsuite/17_intro/headers/c++2020/operator_names.cc: New.
	* testsuite/17_intro/headers/c++2020/stdc++.cc: New.
	* testsuite/17_intro/headers/c++2020/stdc++_multiple_inclusion.cc:
	New.
	* testsuite/18_support/headers/version/macros.cc: New.
	* testsuite/18_support/headers/version/macros.cc: New.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@261706 138bc75d-0d04-0410-961f-82ee72b054a4