LanguageClient: Do not clear shadow documents on reset

We need to preserve this information. Instead, clear only the list of
reverse document dependencies.

Change-Id: I5589d5e3eff613706ea5f2029df1f90eacbbbb4e
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/client.cpp

@@ -1533,7 +1533,8 @@ bool ClientPrivate::reset()
     qDeleteAll(m_documentHighlightsTimer);
     m_documentHighlightsTimer.clear();
     m_progressManager.reset();
-    m_shadowDocuments.clear();
+    for (auto &doc : m_shadowDocuments)
+        doc.second.clear();
     m_documentVersions.clear();
     return true;
 }