ClangCodeModel: Remove libclang fallback for "follow symbol"

... and "switch between declaration/definition".
It's either clangd or built-in code model now.
Use the opportunity to dissolve the pointless FollowSymbolInterface
class hierarchy, which introduced a confusing parallel inheritance
chain.

Change-Id: I792ad55656c5dd9f10c6b4db7c5c36cf7be45125
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cppeditor/cppeditorwidget.cpp

@@ -50,7 +50,6 @@
 #include "cpptoolssettings.h"
 #include "cppuseselectionsupdater.h"
 #include "cppworkingcopy.h"
-#include "followsymbolinterface.h"
 #include "refactoringengineinterface.h"
 #include "symbolfinder.h"
 
@@ -1045,9 +1044,7 @@ void CppEditorWidget::switchDeclarationDefinition(bool inNextSplit)
         if (self && link.hasValidTarget())
             self->openLink(link, split);
     };
-    followSymbolInterface().switchDeclDef(cursor, std::move(callback),
-                                          d->m_modelManager->snapshot(), d->m_lastSemanticInfo.doc,
-                                          d->m_modelManager->symbolFinder());
+    CppModelManager::instance()->switchDeclDef(cursor, std::move(callback));
 }
 
 void CppEditorWidget::findLinkAt(const QTextCursor &cursor,
@@ -1087,13 +1084,10 @@ void CppEditorWidget::findLinkAt(const QTextCursor &cursor,
         }
         callback(link);
     };
-    followSymbolInterface().findLink(
+    CppModelManager::instance()->followSymbol(
                 CursorInEditor{cursor, filePath, this, textDocument()},
                 std::move(callbackWrapper),
                 resolveTarget,
-                d->m_modelManager->snapshot(),
-                d->m_lastSemanticInfo.doc,
-                d->m_modelManager->symbolFinder(),
                 inNextSplit);
 }
 
@@ -1102,11 +1096,6 @@ unsigned CppEditorWidget::documentRevision() const
     return document()->revision();
 }
 
-FollowSymbolInterface &CppEditorWidget::followSymbolInterface() const
-{
-    return d->m_modelManager->followSymbolInterface();
-}
-
 bool CppEditorWidget::isSemanticInfoValidExceptLocalUses() const
 {
     return d->m_lastSemanticInfo.doc && d->m_lastSemanticInfo.revision == documentRevision()