Add 'is_callable' and 'is_nothrow_callable' traits and cleanup INVOKE.

The primary purpose of this patch is to add the 'is_callable' traits.
Since 'is_nothrow_callable' required making 'INVOKE' conditionally noexcept
I also took this oppertunity to implement a constexpr version of INVOKE.
This fixes 'std::experimental::apply' which required constexpr 'INVOKE support'.

This patch will be followed up with some cleanup. Primarly removing most
of "__member_function_traits" since it's no longer used by INVOKE (in C++11 at least).


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@266836 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/bind/func.bind/func.bind.bind/PR23141_invoke_not_constexpr.pass.cpp

@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <functional>
+
+// template<CopyConstructible Fn, CopyConstructible... Types>
+//   unspecified bind(Fn, Types...);
+// template<Returnable R, CopyConstructible Fn, CopyConstructible... Types>
+//   unspecified bind(Fn, Types...);
+
+// https://llvm.org/bugs/show_bug.cgi?id=23141
+#include <functional>
+#include <type_traits>
+
+struct Fun
+{
+  template<typename T, typename U>
+  void operator()(T && t, U && u) const
+  {
+    static_assert(std::is_same<U, int &>::value, "");
+  }
+};
+
+int main()
+{
+    std::bind(Fun{}, std::placeholders::_1, 42)("hello");
+}