[clang] [CGData] Clang Options (PR #90304)

[Kyungwoo Lee via cfe-commits]

kyulee-com wrote:

> Can we add some documentation to https://github.com/llvm/llvm-project/blob/main/clang/docs/UsersManual.rst? This could also be a separate PR.
> 
> LGTM, but I want to give others a chance to review.

Added some documentation, and simplify the flag use by removing the directory case as discussed in #90166.

https://github.com/llvm/llvm-project/pull/90304