ClangTools: Refer to items consistently

Sometimes we used "issue" (treeview header, or filter line edit) and
sometimes "diagnostic" (go to next/pervious diagnostic actions).

Stick to "diagnostic" as it's more generic and avoid conflicts/confusion
with the "Issues pane".

Task-number: QTCREATORBUG-19047
Change-Id: I26b814b117a8fd475c080f0ef79c9fabc1446406
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangtools/clangtoolsdiagnosticmodel.cpp

@@ -84,7 +84,7 @@ ClangToolsDiagnosticModel::ClangToolsDiagnosticModel(QObject *parent)
     : Utils::TreeModel<>(parent)
     , m_filesWatcher(std::make_unique<QFileSystemWatcher>())
 {
-    setHeader({tr("Issue"), tr("Fixit Status")});
+    setHeader({tr("Diagnostic"), tr("Fixit Status")});
     connectFileWatcher();
 }