C++ support is pretty good with base Clang 5, OpenBSD is using the libc++ standard library and libc++abi (FreeBSD uses PathScale's libcxxrt). The situation with debuggers is another story, the base version of gdb is outdated (GPLv2 version). The ports/packages version (egdb) has better support for things like C++, threads, but may lack other features. There is currently no support for the LLVM debugger, sadly.
