AST: Respect alignment attributes on typedef'd arrays

When instantiating an array that has an alignment attribute on it, we
were looking through the array type and only considering the element
type for the resulting alignment. We need to make sure we take the
array's requirements into account too.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206317 91177308-0d34-0410-b5e6-96231b3b80d8