829e2a5526
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.
This directory also contains the Python sources for hdrgen, which is what generates the headers public libc headers. The definitions for these headers are in the ``include`` directory. The ``.h.def`` files are the bases and the ``.yaml`` files are the contents.