Always mark unstable const fn as not const

src/librustc_mir/transform/qualify_consts.rs

@@ -972,13 +972,8 @@ This does not pose a problem by itself because they can't be accessed directly."
  // this doesn't come from a macro that has #[allow_internal_unstable]
  !self.span.allows_unstable()
  {
- if self.mode == Mode::Fn {
- // We are in a normal function
- // with a turned off feature gate. We can still call the function
- // but we can't promote it
- self.qualif = Qualif::NOT_CONST;
- debug!("unstable const fn");
- } else {
+ self.qualif = Qualif::NOT_CONST;
+ if self.mode != Mode::Fn {
  // inside a constant environment, not having the feature gate is
  // an error
  let mut err = self.tcx.sess.struct_span_err(self.span,