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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Do we also want, in a follow-up, to add Visibility to the AST dump?

I do not have as strong of a motivation for that, but I can imagine that happening to help with modules, maybe?
Putting together a quick local patch to dump formal linkage is something I did multiple times over the years, but it never happened with the visibility.

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