CONCEPT_REQUIRES_ preprocesses into something ridiculous for documentation

[timsong-cpp]

int _concept_requires___LINE__ = 42, typename std::enable_if< (_concept_requires___LINE__==43)||(Range< Rng & >()&&Function< Action, Rng &, Rest &&... >()), int >::type = 0 sucks.

[timsong-cpp]

beeb774 prettifies this somewhat.