[Bug c/63591] be more strict when accepting parameter forward declarations

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63591

--- Comment #8 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Andreas Schwab from comment #7)
> A function declaration with forward declared parameters it is a prototype,
> sort of.  Not defining the forward declared parameter as a real parameter
> should probably be flagged as an error.

Indeed. Still someone has to implement it... Kjetil?