[libcxx] Portability fix: make return value of array<T, 0>.data() checked only for libc++.

[amakc11]

Summary

The section array.zero says: "The return value of data() is unspecified". This patch marks all checks of the array<T, 0>.data() return value as libc++ specific.

[ldionne]

Committed as r348485.

test/std/containers/sequences/array/array.data/data.pass.cpp

int main()
         assert(p[1] == 2);
         assert(p[2] == 3.5);
     }
     {
         typedef double T;
         typedef std::array<T, 0> C;
         C c = {};
         T* p = c.data();
-        assert(p != nullptr);
+        LIBCPP_ASSERT(p != nullptr);
     }
     {
       typedef double T;
       typedef std::array<const T, 0> C;
       C c = {{}};
       const T* p = c.data();
       static_assert((std::is_same<decltype(c.data()), const T*>::value), "");
-      assert(p != nullptr);
+      LIBCPP_ASSERT(p != nullptr);
     }
   {
       typedef std::max_align_t T;
       typedef std::array<T, 0> C;
       const C c = {};
       const T* p = c.data();
-      assert(p != nullptr);
+      LIBCPP_ASSERT(p != nullptr);
       std::uintptr_t pint = reinterpret_cast<std::uintptr_t>(p);
       assert(pint % TEST_ALIGNOF(std::max_align_t) == 0);
     }
     {
       typedef NoDefault T;
       typedef std::array<T, 0> C;
       C c = {};
       T* p = c.data();
-      assert(p != nullptr);
+      LIBCPP_ASSERT(p != nullptr);
     }
 }

test/std/containers/sequences/array/array.data/data_const.pass.cpp

int main()
         const T* p = c.data();
         (void)p; // to placate scan-build
     }
     {
       typedef NoDefault T;
       typedef std::array<T, 0> C;
       const C c = {};
       const T* p = c.data();
-      assert(p != nullptr);
+      LIBCPP_ASSERT(p != nullptr);
     }
     {
       typedef std::max_align_t T;
       typedef std::array<T, 0> C;
       const C c = {};
       const T* p = c.data();
-      assert(p != nullptr);
+      LIBCPP_ASSERT(p != nullptr);
       std::uintptr_t pint = reinterpret_cast<std::uintptr_t>(p);
       assert(pint % TEST_ALIGNOF(std::max_align_t) == 0);
     }
 #if TEST_STD_VER > 14
     {
         typedef std::array<int, 5> C;
         constexpr C c1{0,1,2,3,4};
         constexpr const C c2{0,1,2,3,4};
 
         static_assert (  c1.data()  == &c1[0], "");
         static_assert ( *c1.data()  ==  c1[0], "");
         static_assert (  c2.data()  == &c2[0], "");
         static_assert ( *c2.data()  ==  c2[0], "");
     }
 #endif
 }