UBSan: Fix alignment checks emitted in downcasts.

Summary:
UBSan was checking for alignment of the derived class on the pointer to
the base class, before converting. With some class hierarchies, this could
generate false positives.

Added test-case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187948 91177308-0d34-0410-b5e6-96231b3b80d8