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

[Owen Pan via cfe-commits]

owenca wrote:

@boomanaiden154 can we have `clang-format-style-options` rebuilt only when it's outdated, i.e. only if at least one of the following has been updated?
```
clang/include/clang/Format/Format.h
clang/include/clang/Tooling/Inclusions/IncludeStyle.h
clang/docs/ClangFormatStyleOptions.rst
clang/docs/tools/dump_format_style.py
```

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