libstdc++: Disable volatile-qualified std::bind for C++20

libstdc++: Disable volatile-qualified std::bind for C++20

LWG 2487 added a precondition to std::bind for C++17, making
volatile-qualified uses undefined. We still support it, but with a
deprecated warning.

P1065R2 made it explicitly ill-formed for C++20, so we should no longer
accept it as deprecated. This implements that change.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/evolution.xml: Document std::bind API
	changes.
	* doc/xml/manual/intro.xml: Document LWG 2487 status.
	* doc/xml/manual/using.xml: Clarify default value of
	_GLIBCXX_USE_DEPRECATED.
	* doc/html/*: Regenerate.
	* include/std/functional (_Bind::operator()(Args&&...) volatile)
	(_Bind::operator()(Args&&...) const volatile)
	(_Bind_result::operator()(Args&&...) volatile)
	(_Bind_result::operator()(Args&&...) const volatile): Replace
	with deleted overload for C++20 and later.
	* testsuite/20_util/bind/cv_quals.cc: Check for deprecated
	warnings in C++17.
	* testsuite/20_util/bind/cv_quals_2.cc: Likewise, and check for
	ill-formed in C++20.