[v3] Re-implement is_constructible, add is_default_constructible, is_destructible, fix PR 48526

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

Hi!

over the last 2 days or so, Daniel Krugler - having filed in due course 
his Copyright Assignment - finally kindly contributed to the project 
shiny new std::is_constructible, std::is_default_constructible and 
std::is_destructible. Being his first contribution, I helped integrating 
the work in our existing <type_traits> infrastructure. As you can see, I 
also took the occasion to use the new logical helper traits elsewhere, 
add a few more tweaks (remove a rather redundant macro, at this point), 
and also add tests in areas where the testsuite was weak (particularly 
because for some time we could assume shared code was at least exercised 
by the tr1/ testsuite).

I'm going to commit the below, tested x86_64-linux.

For further contributions, I suppose Daniel is going to directly post 
patches to the mailing list, in the normal way in terms of coding 
conventions, testing requirements, etc...

Thanks,
Paolo.

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