[libc++] Simplify the implementation of LWG2762 (noexcept for unique_ptr)

I had originally made some comments in https://reviews.llvm.org/D128214
that were followed when we implemented LWG2762. I don't understand why
I made these comments anymore, but either way it seems like I was wrong
since using `unique_ptr<void>::operator*` should be ill-formed. All other
implementations also make that ill-formed.

Author: ldionne

libcxx/include/__memory/unique_ptr.h

@@ -36,7 +36,6 @@
 #include <__type_traits/is_trivially_relocatable.h>
 #include <__type_traits/is_void.h>
 #include <__type_traits/remove_extent.h>
-#include <__type_traits/remove_pointer.h>
 #include <__type_traits/type_identity.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
@@ -52,17 +51,6 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#ifndef _LIBCPP_CXX03_LANG
-
-template <class _Ptr>
-struct __is_noexcept_deref_or_void {
-  static constexpr bool value = noexcept(*std::declval<_Ptr>());
-};
-
-template <>
-struct __is_noexcept_deref_or_void<void*> : true_type {};
-#endif
-
 template <class _Tp>
 struct _LIBCPP_TEMPLATE_VIS default_delete {
   static_assert(!is_function<_Tp>::value, "default_delete cannot be instantiated for function types");
@@ -266,7 +254,7 @@ class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr {
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 __add_lvalue_reference_t<_Tp> operator*() const
-      _NOEXCEPT_(__is_noexcept_deref_or_void<pointer>::value) {
+      _NOEXCEPT_(_NOEXCEPT_(*std::declval<pointer>())) {
     return *__ptr_.first();
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 pointer operator->() const _NOEXCEPT { return __ptr_.first(); }

libcxx/test/std/utilities/memory/unique.ptr/noexcept_operator_star.compile.pass.cpp

@@ -31,10 +31,6 @@ struct Deleter {
 #endif
 
 TEST_CONSTEXPR_CXX23 bool test() {
-  ASSERT_NOEXCEPT(*(std::unique_ptr<void>{}));
-#if TEST_STD_VER >= 11
-  static_assert(noexcept(*std::declval<std::unique_ptr<void>>()), "");
-#endif
   {
     std::unique_ptr<int> p(new int(3));
     assert(*p == 3);