Make internal class __wrap_iter constexpr when not using libc++'s debugging mode. Introduce a new macro _LIBCPP_CONSTEXPR_IF_NODEBUG to mark this.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337019 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed