[MS-ABI] Implement dynamic_cast
This patch implements call lower from dynamic_cast to __RTDynamicCast
and __RTCastToVoid.  Test cases are included.  A feature of note is that
helper function getPolymorphicOffset is placed in such a way that it can
be used by EmitTypeid (to be implemented in a later patch) without being
moved.  Details are included as comments directly in the code.