Pull in r214736 from upstream libc++ trunk (by Marshall Clow):

  Fix PR#20520 - predicate called too many times in list::remove_if.
  Add tests for list, forward_list, and the std::remove_if algorithm

This fixes an issue where std::list<>::remove_if() and remove() could
erroneously visit elements twice.

Reported by:	Dominic Fandrey <kamikaze@bsdforen.de>
PR:		192303
MFC after:	3 days