PR17576: Fix assertion on polymorphic classes with small alignment

We have to reserve at least the width of a pointer for the vfptr.  For
classes with small alignment, we weren't reserving enough space, and
were overlapping the first field with the vfptr.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192626 91177308-0d34-0410-b5e6-96231b3b80d8