Implement LWG 2148: Make non-enum default hash specialization well-formed

Summary:
This patch removes the static_assert for non-enum types in the primary hash template. Instead non-enum types create a hash<T> specialization that is not constructible nor callable.

See also:
  * http://cplusplus.github.io/LWG/lwg-active.html#2543
  * https://llvm.org/bugs/show_bug.cgi?id=28917

Reviewers: mclow.lists, EricWF

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D23331

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@278300 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/unord.hash/enum.pass.cpp

@@ -7,6 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03, c++11
+
 // <functional>
 
 // make sure that we can hash enumeration values
@@ -14,8 +16,6 @@
 
 #include "test_macros.h"
 
-#if TEST_STD_VER >= 14
-
 #include <functional>
 #include <cassert>
 #include <type_traits>
@@ -59,6 +59,3 @@ int main()
 
     test<Fruits>();
 }
-#else
-int main () {}
-#endif