Simplify initialisation diagnostic when types are cv-qualified

I'd like to revise the diagnostic

error: cannot initialize a variable of type 'const icun_status' with an rvalue of type 'int'

to

error: cannot initialize a constant of type 'icun_status' with an rvalue of type 'int'

The const in this diagnostic doesn't appear to be doing much other than ensuring (unnecessary) technical accuracy. I don't think similar nouns exist for volatile and restrict don't exist: they can remain variable.

I have self-assigned, but this is a good candidate for someone looking to get started with Clang, and I'll happily hand it off in that case.