[clang] Support '-fmodule-file-home-is-cwd' for C++ header units. (PR #135147)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > If you did `-fmodule-file=<name>=<pcm>` for header units, I think it is better to upstream that first. I think it is more fundamental.
> 
> We don't do anything special downstream. As far as I know it already works today.

If `-fmodule-file=<name>=<pcm>` already works for header units, it is helpful to update the documents.

https://github.com/llvm/llvm-project/pull/135147