[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.

llvm-svn: 210377