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/clangcodemodel/clangdclient.h

@@ -126,6 +126,7 @@ private:
     LanguageClient::DiagnosticManager *createDiagnosticManager() override;
     bool referencesShadowFile(const TextEditor::TextDocument *doc,
                               const Utils::FilePath &candidate) override;
+    bool fileBelongsToProject(const Utils::FilePath &filePath) const override;
 
     class Private;
     class VirtualFunctionAssistProcessor;