[CodeGen] Represent array members in new-format TBAA type descriptors

Now that in the new TBAA format we allow access types to be of
any object types, including aggregate ones, it becomes critical
to specify types of all sub-objects such aggregates comprise as
their members. In order to meet this requirement, this patch
enables generation of field descriptors for members of array
types.

Differential Revision: https://reviews.llvm.org/D41399

llvm-svn: 321352