[C++ Patch] Tweak check_previous_goto_1 to emit hard errors instead of permerrors in some cases

[Paolo Carlini]

.. an additional clarification (I told you that over the years we 
changed this code quite a bit...): I originally added the testcase that 
I'm adjusting here, I did that when, back in 2014, I worked on 63558: 
the test uses -fpermissive -w and was meant to check, as requested by 
Manuel in the bug, that we didn't issue further unsuppressible 
explanatory diagnostic after the primary permerror. I would argue that 
that first clean-up back 2014 was an improvement but note that, in 
practice, before it we rejected (with a confusing permerror + error) 
this kind of broken code and we don't anymore. That's bad. We don't 
really emit sensible assembly for it, as I already explained.

Paolo.