LanguageClient: Add signal to inform about WorkDoneProgressEnd

... and make use of it in the clangd client to be able tell when
background indexing has finished.

Change-Id: I0f3c6f9646fd66ababd08c12b2f347da5f1a3729
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdclient.h

@@ -37,6 +37,11 @@ class ClangdClient : public LanguageClient::Client
     Q_OBJECT
 public:
     ClangdClient(ProjectExplorer::Project *project, const Utils::FilePath &jsonDbDir);
+
+    bool isFullyIndexed() const { return m_isFullyIndexed; }
+
+private:
+    bool m_isFullyIndexed = false;
 };
 
 } // namespace Internal