ClangCodeModel: Implement per-session mode for clangd

Fixes: QTCREATORBUG-26526
Change-Id: If9e018475b4e2f0557d9bf64ad9a7921c9dd6046
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/client.cpp

@@ -2051,6 +2051,11 @@ bool Client::referencesShadowFile(const TextEditor::TextDocument *doc,
     return false;
 }
 
+bool Client::fileBelongsToProject(const Utils::FilePath &filePath) const
+{
+    return project() && project()->isKnownFile(filePath);
+}
+
 } // namespace LanguageClient
 
 #include <client.moc>