[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/empty.fail.cpp

@@ -23,6 +23,9 @@
 
 int main ()
 {
+
     std::array<int, 1> c;
-    c.empty();  // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+    c.empty(); // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
+    std::array<int, 0> c0;
+    c0.empty(); // expected-error {{ignoring return value of function declared with 'nodiscard' attribute}}
 }