Correctly diagnose prototype redeclaration errors in C

We did not implement C99 6.7.5.3p15 fully in that we missed the rule
for compatible function types where a prior declaration has a prototype
and a subsequent definition (not just declaration) has an empty
identifier list or an identifier list with a mismatch in parameter
arity. This addresses that situation by issuing an error on code like:

void f(int);
void f() {} // type conflicts with previous declaration

(Note: we already diagnose the other type conflict situations
appropriately, this was the only situation we hadn't covered that I
could find.)