[MS ABI] Tolerate invokes of __RTDynamicCast

The pointer returned by __RTDynamicCast must be bitcasted.  However, it
was not expected that __RTDynamicCast would be invoked, resulting in the
bitcast occuring in a different BasicBlock than the invoke.  This caused
a down-stream PHI to get confused about which BasicBlock the incomming
value was from.

This fixes PR25606.

llvm-svn: 253843