[v3 patch] Implement N4502 Standard Library Support for the C++ Detection Idiom

[Jonathan Wakely]

This adds the "detection idiom" API from N4502

http://open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf

I've added the low-level parts to <type_traits>, with public aliases
in <experimental/type_traits>, so that I can use them elsewhere in the
library.

Tested powerpc64le-linux, committed to trunk.
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