Add symlink from /usr/include/c++/v1 -> /usr/lib/llvm-N/include/c++/v1

This is needed for separately packaged tools (including clang) to find libc++ without extra configuration.

The global directory was lost with the libc++-dev => libc++-N-dev switch. Today only tools installed under /usr/lib/llvm-N/bin will find libc++, which doesn't make sense as libc++/clang versions aren't tightly coupled.

Currently different major versions libc++-N-dev conflict, if coinstallation is supported then I expect the symlink could be managed by update-alternatives, in parallel to libc++.so.