ADT/SparseMultiSetTest.cpp: Try to appease cygwin-clang (libstdc++-4.5) on stage2. [-Wsign-compare]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173127 91177308-0d34-0410-b5e6-96231b3b80d8