[v3] Update std::forward to N3092

[Paolo Carlini]

... apparently US 90 is actually getting support, thus to avoid ping -
pongs between different versions, I'm reinstating the N2835 version of
std::forward which we have in 4_5-branch too (with common_type replacing
of course identity, because the latter is gone). Tested x86_64-linux.

Paolo.

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