[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

llvm-svn: 324526

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

@@ -38,6 +38,25 @@ int main()
         const T* p = c.data();
         (void)p; // to placate scan-build
     }
+    {
+      struct NoDefault {
+        NoDefault(int) {}
+      };
+      typedef NoDefault T;
+      typedef std::array<T, 0> C;
+      const C c = {};
+      const T* p = c.data();
+      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);
+    }
 #if TEST_STD_VER > 14
     {
         typedef std::array<int, 5> C;