SCARY/N2913 iterator support between the multi and non-multi versions of the...

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.

llvm-svn: 191692