[clang] [clang-format][CMake] Generate formatting options docs during build (PR #113739)

[Owen Pan via cfe-commits]

owenca wrote:

> What's the point of adding it to the `FormatTests` target though? Just to ensure proper test coverage of the `dump_format_style.py` file?

That and to also ensure that the edited Format.h doesn't break the python script.

> Thinking about this a bit more, I think we can probably satisfy all the constraints if we write a lit test that ensures the in-tree ClangFormatStyleOptions.rst matches what the script will produce (https://github.com/llvm/llvm-project/pull/118154)? That solves the original problem that https://github.com/llvm/llvm-project/pull/111513 set out to solve (if I remember correctly) and will show up as a failure in CI if not done.

If we are to leave the generated part of the rst file in the repo, #111513 would satisfy my requirements nicely. Something like #118154 may be a useful addition.

> I'm also planning on making the documentation build action upload the built docs as artifacts, which would also let reviewers just download the built HTML files and inspect those if that's easier/useful.

That would be nice.

https://github.com/llvm/llvm-project/pull/113739