[C++ Patch] Avoid pairs of error calls in duplicate_decls

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

Hi,

thus, as agreed a few days ago, I audited duplicate_decls for those 
pairs of error and permerror where we want to use inform. I found at 
least a straightforward case having to do with pragma omp, but other 
cases required a little more work (I suspected that). For example in 
some cases we issued pairs of errors with the same location (which I 
merged); or we mentioned in the diagnostics the olddecl *before* the 
newdecl (we want the inform about the olddecl). In some cases we had 
inform (input_location, ...) and then '+', which seems pretty silly to 
me. Finally, for consistency, I used 'ambiguating' similarly to 
'conflicting' in at least a message, not sure if we could do better, 
English-wise.

Tested x86_64-linux.

Thanks,
Paolo.

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