[clang] [clang-tools-extra] [clang] Add formal linkage to textual AST dump (PR #194600)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

> > Do we also want, in a follow-up, to add Visibility to the AST dump?
> 
> @mizvekov working on #198215, I realized how useful it would be to also have visibility in AST dumps, so I intend to follow this up with a visibility patch. But before that, I'm going to work on AST dump tests to make the visibility PR much less painful than this one.

Great, thanks for that!

Yeah, I had to debug a couple of visibility issues lately and that would have been useful.

https://github.com/llvm/llvm-project/pull/194600