libstdc++: Replace std::result_of with __invoke_result_t [PR114394]

Replace std::result_of with std::invoke_result, as specified in the
standard since C++17, to avoid deprecated warnings for std::result_of.

We don't have __invoke_result_t in C++11 mode, so add it as an alias
template for __invoke_result<>::type (which is what std::result_of uses
as its base class, so there's no change in functionality).

This fixes warnings given by Clang 18.

libstdc++-v3/ChangeLog:

	PR libstdc++/114394
	* include/std/functional (bind): Use __invoke_result_t instead
	of result_of::type.
	* include/std/type_traits (__invoke_result_t): New alias
	template.
	* testsuite/20_util/bind/ref_neg.cc: Adjust prune pattern.