[libc] Revamp hdrgen command line and build integration (#121522)
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.
llvm-project
6ad0dcf6 - [libc] Revamp hdrgen command line and build integration (#121522)
