Correct comment regarding members of __cxa_dependent_exception. Actually it...

Correct comment regarding members of __cxa_dependent_exception.  Actually it is useful to disguise a __cxa_dependent_exception as a __cxa_exception by filling in most (not all) of these members at __cxa_dependent_exception construction time.  That way most routines don't have to care which kind of exception they have.

llvm-svn: 147759