`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.
This adds a new main command-line entry point for hdrgen, in the
new main.py. This new interface is used for generating a header.
The old ways of invoking yaml_to_classes.py for other purposes
are left there for now, but `--e` is renamed to `--entry-point`
for consistency with the new CLI.
The YAML schema is expanded with the `header_template` key where
the corresponding `.h.def` file's path is given relative to where
the YAML file is found. The build integration no longer gives
the `.h.def` path on the command line. Instead, the script now
emits a depfile that's used by the cmake rules to track that.
The output file is always explicit in the script command line
rather than sometimes being derived from a directory path.
The .yaml files should live next to the corresponding .h.def
files in libc/include/, rather than next to the implementation of
the tool in libc/utils/hdrgen/. As with the .h.def files, there
is no need for a yaml/ subdirectory under include/. This simpler
layout is more natural for maintenance and also simplifies build
integration outside the LLVM CMake build.
