[v3] more constexpr hacking

[Benjamin De Kosnik]

.... as per LWG comments on N330[2-5].

Couple of notes: new fail has been temporarily worked around pending
the resolution of 

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54968

For <tuple>, the big fun remaining is piecewise_construct_t, which
should allow tie and forward_as_tuple to be constexpr. I'm too tired to
get into this with -fconstexpr-depth=2 tonight.

Of course, if Daniel or Jonathan want to hack on this while I'm asleep,
well then....

tested x86/linux

-benjamin
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