[clang] Support C++20 Modules in clang-repl (PR #79261)

[Balazs Benics via cfe-commits]

steakhal wrote:

> Do you mean the documentation? If so, yes, that’s probably not the right place. I am on my phone but can you suggest a place where we should move this or just move it? I think that was an oversight.

Thanks. There is nothing urgent. I was just preparing a PR for syncing the release notes with what we actually did in CSA, so I realized this.

I wanted to check first if you already have a well established place/section where you would prefer to have this, as I couldn't find one in 3 seconds.
You can move it, or alternatively I can also move it.
It's up to you. :)

(The release branch is [supposed](https://discourse.llvm.org/t/llvm-19-release-schedule-and-planning/79828?u=steakhal) to branch off on the 23rd of July)

https://github.com/llvm/llvm-project/pull/79261