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.

Performed en-masse via:

    $ grep -rn "#include <ll" libc/include -l | \
      xargs perl -pi -e 's/#include <ll(.*)>/#include "ll$1"/'
    $ grep -rn "#include <__" libc/include -l | \
      xargs perl -pi -e 's/#include <__(.*)>/#include "__$1"/'

Link: #83463
Link: #83210

Per spec:
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/htonl.html
* https://pubs.opengroup.org/onlinepubs/9699919799/functions/htons.html

Also adds UInt16Type and UInt32Type to spec.td

Co-authored-by: Jeff Bailey <jbailey@google.com>

Reviewed By: sivachandra, jeffbailey, rtenneti

Differential Revision: https://reviews.llvm.org/D143795