[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/at.pass.cpp

@@ -56,6 +56,26 @@ int main()
         catch (const std::out_of_range &) {}
 #endif
     }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        typedef double T;
+        typedef std::array<T, 0> C;
+        C c = {};
+        C const& cc = c;
+        try
+        {
+            TEST_IGNORE_NODISCARD  c.at(0);
+            assert(false);
+        }
+        catch (const std::out_of_range &) {}
+        try
+        {
+            TEST_IGNORE_NODISCARD  cc.at(0);
+            assert(false);
+        }
+        catch (const std::out_of_range &) {}
+    }
+#endif
     {
         typedef double T;
         typedef std::array<T, 3> C;