Add a specialization for '__unwrap_iter' to handle const interators. This...

Add a specialization for '__unwrap_iter' to handle const interators. This enables the 'memmove' optimization for std::copy, etc.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353311 91177308-0d34-0410-b5e6-96231b3b80d8