Fix 61347 testcase in alternative modes

[François Dumont]

Hi

     I can't find how to fix 61347 for alternative modes. I started with 
debug mode by providing a forward declaration for safe iterator type 
which return to the __distance on normal iterator but gcc still doesn't 
consider it as a builtin constant.

     I guess the problem might come from compilations options, so is it 
that we should simply add a dg-require-normal-mode to have this test 
unsupported ? Otherwise any advise to fix this test ?

Thanks

François

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