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

[Owen Pan via cfe-commits]

owenca wrote:

> Can you explicate on what your requirements are here? I don't see why just wrapping the python script in CMake does anything tangible here. Having a build target that modifies the source directory is also weird and something that we should avoid if possible, in my opinion.

>From https://github.com/llvm/llvm-project/pull/111513#issuecomment-2408412143:
> Anyway, my current workflow is:
> 
> 1. Edit files in clang/lib/Format and clang/unittests/Format.
> 2. Edit clang/include/clang/Format/Format.h if needed.
> 3. Run ninja FormatTests.
> 4. Run cd clang/docs/tools && dump_format_style.py if Step 2 above wasn't skipped.
> 5. Run the unit tests.
> 6. Run git commit -a.
> 
> It would be nice if I didn't have to run step 4 manually.

With #111513, I no longer need to remind myself to run the python script when testing a patch. Please note that we rarely need to do `ninja check-clang-format` to run the lit tests, which takes an extra hour or two.

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