[libcxx] P0604, invoke_result and is_invocable

Summary:
Introduce a new form of `result_of` without function type encoding.

Rename and split `is_callable/is_nothrow_callable` into `is_invocable/is_nothrow_invocable/is_invocable_r/is_nothrow_invocable_r` (and associated types accordingly)

Change function type encoding of previous `is_callable/is_nothrow_callable` traits to conventional template type parameter lists.


Reviewers: EricWF, mclow.lists, bebuch

Reviewed By: EricWF, bebuch

Subscribers: lichray, bebuch, cfe-commits

Differential Revision: https://reviews.llvm.org/D38831

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@320509 91177308-0d34-0410-b5e6-96231b3b80d8

include/variant

@@ -582,7 +582,7 @@ struct __variant {
 private:
   template <class _Visitor, class... _Values>
   static constexpr void __std_visit_exhaustive_visitor_check() {
-    static_assert(is_callable_v<_Visitor(_Values...)>,
+    static_assert(is_invocable_v<_Visitor, _Values...>,
                   "`std::visit` requires the visitor to be exhaustive.");
   }