[clang 17456] reference cast diagnostic for unrelated classes

[Nathan Sidwell]

Richard,
here's the reference-cast case of the unrelated hierarchy patch I recently 
created for ptr-to-class (http://llvm.org/bugs/show_bug.cgi?id=17456).  I had a 
bit of a struggle coming up with a nicely worded diagnostic.  This now produces:

17456-2.cc:6:10: error: non-const lvalue reference to class 'B' cannot bind to a 
value of class 'A', which is not related by inheritance
   return static_cast<B&>(arg);
          ^               ~~~
when the src and target reference types are classes.  It also notes if either is 
incomplete.

ok?

nathan
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