Optimize emission of `dynamic_cast` to final classes.

- When the destination is a final class type that does not derive from
  the source type, the cast always fails and is now emitted as a null
  pointer or call to __cxa_bad_cast.

- When the destination is a final class type that does derive from the
  source type, emit a direct comparison against the corresponding base
  class vptr value(s). There may be more than one such value in the case
  of multiple inheritance; check them all.

For now, this is supported only for the Itanium ABI. I expect the same thing is
possible for the MS ABI too, but I don't know what guarantees are made about
vfptr uniqueness.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D154658