[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.cons/default.pass.cpp

@@ -14,6 +14,14 @@
 #include <array>
 #include <cassert>
 
+// std::array is explicitly allowed to be initialized with A a = { init-list };.
+// Disable the missing braces warning for this reason.
+#include "disable_missing_braces_warning.h"
+
+struct NoDefault {
+  NoDefault(int) {}
+};
+
 int main()
 {
     {
@@ -28,4 +36,13 @@ int main()
         C c;
         assert(c.size() == 0);
     }
+    {
+      typedef std::array<NoDefault, 0> C;
+      C c;
+      assert(c.size() == 0);
+      C c1 = {};
+      assert(c1.size() == 0);
+      C c2 = {{}};
+      assert(c2.size() == 0);
+    }
 }