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

[schwab@linux-m68k.org]

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

--- Comment #7 from Andreas Schwab <schwab@linux-m68k.org> ---
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.