Fix initialization of array<const T, 0> with GCC.

Previously, when handling zero-sized array of const objects we
used a const version of aligned_storage_t, which is not an array type.
However, GCC complains about initialization of the form: array<const T, 0> arr = {};

This patch fixes that bug by making the dummy object used to represent
the zero-sized array an array itself. This avoids GCC's complaints
about the uninitialized const member.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324194 91177308-0d34-0410-b5e6-96231b3b80d8