Create a 'comma_iterator' class that overloads operator, and asserts when it's called. Add tests to mismatch to make sure it can't be blindsided by such an evil iterator. More tests for other algorithms forthcoming. Thanks to STL for pointing this out at CppCon and Yakov Galka for opening LWG issue #2133

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217902 91177308-0d34-0410-b5e6-96231b3b80d8