[CodeGen] Handle flexible array members containing pointers to members

[CodeGen] Handle flexible array members containing pointers to members

Types can be classified as being zero-initializable or
non-zero-initializable.  We used to classify array types by giving them
the classification of their base element type.  However, incomplete
array types are never initialized directly and thus are always
zero-initializable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@238256 91177308-0d34-0410-b5e6-96231b3b80d8