[Bug c++/67371] Never executed "throw" in constexpr function fails to compile

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67371

--- Comment #6 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
Well, my patch survives regression testing and fixes the issue.

But unfortunately it leads to accept invalid cases like:

constexpr int f1() {
  throw;
  return 0;
}

or 

constexpr void f2() {
  throw;
}

(Even clang accepts f2.)