Fix size and alignment of array<T, 0>.

An array T[1] isn't necessarily the same say when it's
a member of a struct. This patch addresses that problem and corrects
the tests to deal with it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324545 91177308-0d34-0410-b5e6-96231b3b80d8