PR66284 remove std::function special case for reference_wrapper

PR66284 remove std::function special case for reference_wrapper

	PR libstdc++/66284
	* doc/xml/manual/intro.xml: Document LWG 2781 change.
	* doc/html/*: Regenerate.
	* include/std/functional (_Function_base::_Ref_manager): Remove.
	(_Function_handler): Remove partial specializations for
	reference_wrapper.
	(function::target): Remove special case for const qualification.
	* testsuite/20_util/function/6.cc: Adjust tests for target type.
	* testsuite/20_util/function/7.cc: Likewise.
	* testsuite/20_util/function/8.cc: Likewise.

From-SVN: r244364