Also add <stop_token> header to PCH and Doxygen config.

	* doc/doxygen/user.cfg.in: Add <stop_token>.
	* include/precompiled/stdc++.h: Likewise.
	* include/std/stop_token: Fix definition of std::nostopstate.
	* testsuite/30_threads/headers/stop_token/synopsis.cc: New test.
	* testsuite/30_threads/headers/thread/types_std_c++20.cc: New test.
	* testsuite/30_threads/stop_token/stop_source.cc: New test.
	* testsuite/30_threads/stop_token/stop_token.cc: Remove unnecessary
	dg-require directives. Remove I/O and inclusion of <iostream>.

From-SVN: r278296