[Bug c++/69855] Missing diagnostic for overload that only differs by return type

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69855

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Minimal reproducer:

int get();
void f() {
  char get();
}

Slightly sneakier version that defeated my first attempt to fix it (because the
overload set causes us to discard the previous decls):

int get();
char get(int);
void f() {
  char get();
}