Add hint for Cpp1 prefix operators

[ntrel]

I'm assuming no Cpp2 expression starts with one of those tokens.

[JohelEGP]

Resolves #416.

[hsutter]

Thanks!

I'll merge this and apply the following updates separately, but for future reference:

• Regression tests that are intended to fail should be named "*-error.cpp2" so the quick sanity-check counts in my run-tests script account for them correctly
• Just a nit: These days I try to keep messages of the form " is not allowed, use instead" or " may not , did you mean ?", though I'm sure I violate that all over the place myself esp. with earlier messages

Again, thanks.