[clang] [clang][ssaf] Add EntityLinkage data structure (PR #181718)

[Balázs Benics via cfe-commits]

steakhal wrote:

> > If you are there could you trim the commit message. I don't believe that you wrote it. It's always AI that comes up with such bloated commit messages.
> 
> Actually, I did write it by hand :-D But I trimmed it down.

Oh, then I sincerely apologise.

https://github.com/llvm/llvm-project/pull/181718