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

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

over the years we reworked and improved the code in decl.c checking 
gotos quite a bit. Lately, in some specific unsafe cases, identify_goto 
issues upfront an error instead of a permerror, whereas it used to 
always issue a permerror. Over the last weeks a few colleagues of mine 
noticed that we don't do that, escalating a permerror to a plain error, 
in a case which is certainly unsafe - decl_jump_unsafe returns 2 - thus, 
if the user passes -fpermissive we end up emitting assembly completely 
missing labels. The straightforward patchlet below passes testing on 
x86_64-linux.

Thanks, Paolo.

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