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

[Matheus Izvekov via cfe-commits]

================
@@ -1429,6 +1429,30 @@ static void dumpBasePath(raw_ostream &OS, const CastExpr *Node) {
   OS << ')';
 }
 
+void TextNodeDumper::dumpFormalLinkage(const NamedDecl *ND) {
+  switch (ND->getFormalLinkage()) {
+  case Linkage::None:
+    // A lot of declarations have no linkage, so we only dump linkage if there
+    // is one.
+    break;
+  case Linkage::Internal:
+    OS << " internal-linkage";
+    break;
+  case Linkage::External:
+    OS << " external-linkage";
+    break;
+  case Linkage::Module:
+    OS << " module-linkage";
+    break;
+  case Linkage::Invalid:
+    OS << " invalid-linkage";
+    break;
----------------
mizvekov wrote:

```suggestion
  case Linkage::Invalid:
    llvm_unreachable("Linkage hasn't been computed!");
```

I don't think this can be observed over an AST dump.

This is used internally to mark declarations that haven't had their linkage computed yet.

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