Initial implementaiton of __dynamic_cast. There is still lots of debugging...

Initial implementaiton of __dynamic_cast.  There is still lots of debugging code in here that needs to be stripped out.  And many, many unit tests need to be written.  And comments and probably code cleanliness needs to be improved.  But I *think* the basic algorithm is sound.  There also may still be some oportunities for algorithm optimization, I'm not positive.

llvm-svn: 147981