Improve diagnostic message for C-style type syntax

Supplemental PR for #4021
Sigh. Even more random flags without any documentation. I really wish you'd take a good look at the advice found in Code Complete or any similar book on how to write readable code. The amount of work you do for D is awesome beyond words, but changes like this, where the only hint what a cryptically named flag scattered throughout the parser might do is in the implementation of a single function, make it harder for everybody else to contribute to the code base.