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

[Aiden Grossman via cfe-commits]

boomanaiden154 wrote:

> can we have clang-format-style-options rebuilt only when it's outdated, i.e. only if at least one of the following has been updated?

We can, but it requires more changes that I'm not sure are worth doing given that this step takes essentially zero time compared to regenerating the docs with Sphinx, which already does run every time.

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