[patch] LWG DR 2285: std::make_reverse_iterator

[Jonathan Wakely]

This is a trivial addition, implementing
http://cplusplus.github.io/LWG/lwg-defects.html#2285

I've also added the __cpp_lib_tuple_element_t macro missing from the
commit that implemented that.

Tested x86_64-linux, committed to trunk.
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