SCARY/N2913 iterator support between the multi and non-multi versions of the associative and unordered containers.  I beleive lack of support for this was accidentally recently introduced (by me) and this is fixing a regression.  This time tests are put in to prevent such a regression in the future.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@191692 91177308-0d34-0410-b5e6-96231b3b80d8