[libc++] Fix PR35491 - std::array of zero-size doesn't work with non-default constructible types.

Summary:
This patch fixes llvm.org/PR35491 and LWG2157  (https://cplusplus.github.io/LWG/issue2157)

The fix attempts to maintain ABI compatibility by replacing the array with a instance of `aligned_storage`.

Reviewers: mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: lichray, cfe-commits

Differential Revision: https://reviews.llvm.org/D41223

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324526 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -13,6 +13,7 @@
 
 #include <array>
 #include <cassert>
+#include "test_macros.h"
 
 // std::array is explicitly allowed to be initialized with A a = { init-list };.
 // Disable the missing braces warning for this reason.
@@ -34,6 +35,33 @@ int main()
         typedef std::array<T, 0> C;
         C c = {};
         T* p = c.data();
-        (void)p; // to placate scan-build
+        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);
+    }
+  {
+      typedef std::max_align_t T;
+      typedef std::array<T, 0> C;
+      const C c = {};
+      const T* p = c.data();
+      assert(p != nullptr);
+      std::uintptr_t pint = reinterpret_cast<std::uintptr_t>(p);
+      assert(pint % TEST_ALIGNOF(std::max_align_t) == 0);
+    }
+    {
+      struct NoDefault {
+        NoDefault(int) {}
+      };
+      typedef NoDefault T;
+      typedef std::array<T, 0> C;
+      C c = {};
+      T* p = c.data();
+      assert(p != nullptr);
     }
 }