C++: const_iterator not comparable to iterator for map<>s

[Benjamin Kosnik]

> | I suspect we'll need two files. We need both positive (should compile) 
> | and negative (should not compile) tests.
> 
> Hmm, if we're going to use DejaGnu shouldn't it suffice to mark FAIL
> or PASS, or am I being dense?

no, you're right, I'm being dense. I'll check it in, and when we convert 
to dejagnu then I'll make sure it's working as expected.

-benjamin