[C++ Patch] Do not warn about [[nodiscard]] applied to a constructor

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

Hi,

in Cologne, during the presentation of P1771R0, Per Sommerlad pointed 
out that apparently GCC was already almost doing the right thing - it 
accepts [[nodiscard]] on a constructor and then a warning is emitted in 
the relevant potentially dangerous situations - but it does first emit a 
warning when it encounters the [[nodiscard]] itself. Avoiding the latter 
seems easy to me - the below passes testing. Something else?

Thanks, Paolo.

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