This adds a feature to hdrgen to emit JSON summaries of header
files for build system integration. For now the summaries have
only the basic information about each header that is relevant for
build and testing purposes: the standards and includes lists.
This makes hdrgen emit `#include "..."` lines using the correct
relative path (number of `../` steps, if any) for the containing
header, and includes this in the sorted block of `#include` lines
also used for the macro headers.
A macro can specify macro_header instead of macro_value to
indicate that an llvm-libc-macros/ header file is supposed to
define this macro. This is used for dlfcn.h, which previously
bogusly redefined the RTLD_* macros to empty.
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.
