ref:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/strcasecmp_l.html

This patch introduces the `strcasecmp_l` function. At present, the
locale parameter is ignored, making it a stub implementation. This is
consistent with how other locale-related functions, such as `islower_l`,
are treated in our codebase as well as in
[musl](https://github.com/bminor/musl/blob/master/src/string/strcasecmp.c)
and
[bionic](https://cs.android.com/android/platform/superproject/main/+/main:bionic/libc/bionic/strings_l.cpp).

---------

Co-authored-by: Michael Jones <michaelrj@google.com>

This patch adds the `strings/ffs` function.
ref: https://pubs.opengroup.org/onlinepubs/9799919799/functions/ffs.html
Closes: #122054.

A number of functions in strings.h take size_t as an argument.

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.