[Bug c++/126420] Rejects valid? deducing return type with expansion statement (empty)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126420

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
A related example is this:
```
int intreturn();
void foo2() {
  template for (constexpr auto x : {}) {
    extern float intreturn();
  }
}
```

clang accepts this while GCC rejects it.