[v3] Add constexpr specifiers to std::shared_ptr, std::unique_ptr, etc

[Paolo Carlini]

Hi,

> We still have a regression in this program:

I'm not sure to understand what you mean by 'still a regression': to be honest I didn't mean to either fix regressions or causing regressions. If you think my patch has made the situation worse in *any* sense, just let me know I will immediately revert it, no problem at all.

Paolo