This patch adds the implementation for `inet_aton` function. Since this
function is not explicitly included in POSIX, I have marked it with
`llvm_libc_ext`. It is widely available and commonly used, and can also
be used to implement `inet_addr`, which is included in POSIX.

This PR fixes broken links in all files describing libc usage modes.
Please let me know if there are any other places that need updating.

---------

Co-authored-by: shubhp@perlmutter <shubhp@perlmutter.com>

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Adds l64a, which generates the base 64 string expected by a64l.

Move the hdrgen code under a subdirectory to treat it as a Python
module.

This mimics the structure used by llvm/utils/lit and
llvm/utils/mlgo-utils and simplifies integration of hdrgen to the build
system which rely on Python modules. In addition to that, it clarifies
which imports are coming from the hdrgen-specific helpers (e.g. "from
type import ..." becomes "from hdrgen.type import ...".

Leave the entrypoints (top-level main.py and yaml_to_classes.py) as-is:
they can keep being referred by the CMake build system w/o any changes.

Implements the posix-specified strftime conversions for the default
locale, along with comprehensive unit tests. This reuses a lot of design
from printf, as well as the printf writer.

Roughly based on #111305, but with major rewrites.

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.