ClangCodeModel: Implement following a symbol to its type

Making use of LSP's "Go To Type Definition".
Just the backend for now, UI to follow.

Change-Id: Id73b2cf701eab03913477f6d4d3093e257e80dbd
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/clangcodemodel/clangdswitchdecldef.cpp

@@ -169,7 +169,7 @@ void ClangdSwitchDeclDef::Private::handleDeclDefSwitchReplies()
     const QTextCursor funcNameCursor = cursorForFunctionName(*functionNode);
     if (!funcNameCursor.isNull()) {
         client->followSymbol(document.data(), funcNameCursor, editorWidget, callback,
-                             true, false);
+                             true, FollowTo::SymbolDef, false);
     }
     q->emitDone();
 }