[clang] [Clang] support friend declarations with a dependent nested-name-specifier (PR #191268)

[Corentin Jabot via cfe-commits]

================
@@ -191,9 +129,10 @@ class FriendDecl final
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
-  static bool classofKind(Kind K) { return K == Decl::Friend; }
+  static bool classofKind(Kind K) {
+    return K == Decl::Friend || K == Decl::FriendTemplate;
+  }
----------------
cor3ntin wrote:

Hum, I think we are both wrong.
Having FriendDecl and FriendTemplateDecl form a hierarchy makes sense. Im just commenting on your implementation of `classofKind`.

But my feedback was wrong.

So, take 2.

You forgot to modify `DeclNodes.td`

```cpp
def Friend : DeclNode<Decl>;
def FriendTemplate : DeclNode<Decl>;
```

should be

```
def Friend : DeclNode<Decl>;
def FriendTemplate : DeclNode<Friend>;
```

I'm not sure what the implication of not doing that are but it can't be anything good!

Once you do that, and after rebuilding, you should have  `Decl::firstFriend` and `Decl::lastFriend` enumerators. The implementation can then be

```cpp
static bool classofKind(Kind K) { return K >= firstFriend && K <= lastFriend; }
```

Which is a bit more robust.

see `firstNamed` or `firstFunction` for examples

https://github.com/llvm/llvm-project/pull/191268