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

[Owen Pan via cfe-commits]

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 can we have clang-format-style-options rebuilt only when it's outdated, i.e. only if at least one of the following has been updated?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

I'm not concerned about how much time it takes to run. I want to add it to `clangFormat`'s dependencies if it doesn't run every time I do `ninja FormatTests`. Can something similar to https://github.com/llvm/llvm-project/pull/111513/files#r1832152957 be done here?

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