ubsan: Add checking for invalid downcasts. Per [expr.static.cast]p2 and p11,
base-to-derived casts have undefined behavior if the object is not actually an
instance of the derived type.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175078 91177308-0d34-0410-b5e6-96231b3b80d8