libstdc++: Improve diagnostics for std::packaged_task invocable checks

Moving the static_assert that checks is_invocable_r_v into _Task_state
means it is checked when we instantiate that class template.

Replacing the __create_task_state function with a static member function
_Task_state::_S_create ensures we instantiate _Task_state and trigger
the static_assert immediately, not deep inside the implementation of
std::allocate_shared. This results in shorter diagnostics that don't
show deeply-nested template instantiations before the static_assert
failure.

Placing the static_assert at class scope also helps us to fail earlier
than waiting until when the _Task_state::_M_run virtual function is
instantiated. That also makes the diagnostics shorter and easier to read
(although for C++11 and C++14 modes the class-scope static_assert
doesn't check is_invocable_r, so dangling references aren't detected
until _M_run is instantiated).

libstdc++-v3/ChangeLog:

	* include/std/future (__future_base::_Task_state): Check
	invocable requirement here.
	(__future_base::_Task_state::_S_create): New static member
	function.
	(__future_base::_Task_state::_M_reset): Use _S_create.
	(__create_task_state): Remove.
	(packaged_task): Use _Task_state::_S_create instead of
	__create_task_state.
	* testsuite/30_threads/packaged_task/cons/dangling_ref.cc:
	Adjust dg-error patterns.
	* testsuite/30_threads/packaged_task/cons/lwg4154_neg.cc:
	Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

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