Add a simple new warning to catch blatantly dangling pointer and

reference members of classes. We've had several bugs reported because of
this, and there's no reason not to flag it right away in the compiler.

Comments especially welcome on the strategy for implementing this
warning (IE, what should trigger this?) and on the text of the warning
itself.

I'm going to extend this to cover obvious cases with temporaries and
beef up the test cases some in subsequent patches. I'll then run it over
a large codebase and make sure its not misbehaving before I add it to
-Wall or turn it on by default. I think this one might be a good
candidate for on by default.

llvm-svn: 139075