Remove aborts under __dynamic_cast which were under _LIBCXX_DYNAMIC_FALLBACK. ...

Remove aborts under __dynamic_cast which were under _LIBCXX_DYNAMIC_FALLBACK.  Change all type_info comparisons to use an inlined is_equal helper.  However no change in functionality for this latter change at this time.  This is just to encapsulate the comparison and make it a little easier to switch back and forth for testing/debugging.

llvm-svn: 168052