[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?

@mizvekov working on https://github.com/llvm/llvm-project/pull/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.

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