`sockaddr_un` structure is supposed to be provided by the `<sys/un.h>`
header. Add this header to llvm-libc, and move the declaration of
`sockaddr_un` there from `<sys/socket.h>`. See
https://man7.org/linux/man-pages/man0/sys_un.h.0p.html
Add proxy headers for the `<sys/socket.h>` macro (like `AF_UNIX`) and
for the `struct sockaddr_un` so that the tests can be more hermetic and
avoid system header inclusion.
3fb55b6872