Debug mode for unordered_multiset.  The exercise spotted a few places I had missed on unordered_set, so I picked those up as well.

There are actually two debug modes:

   1.  -D_LIBCPP_DEBUG2 or -D_LIBCPP_DEBUG2=1
       This is a relatively expensive debug mode, but very thorough.  This is normally what you want to debug with, but may turn O(1) operations into O(N) operations.

   2.  -D_LIBCPP_DEBUG2=0
       This is "debug lite."  Only preconditions that can be checked with O(1) expense are checked.  For example range checking on an indexing operation.  But not iterator validity.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187369 91177308-0d34-0410-b5e6-96231b3b80d8