ClangCodeModel: Provide source of diagnostics in tooltip

It's helpful to know whether a particular issue came from clangd,
libclang, or somewhere else.

Change-Id: I9e00d79ffe630a91028c2aeff219571e9979dbee
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangeditordocumentprocessor.cpp

@@ -506,7 +506,8 @@ ClangEditorDocumentProcessor::creatorForHeaderErrorDiagnosticWidget(
         vbox->setSpacing(2);
 
         vbox->addWidget(ClangDiagnosticWidget::createWidget({firstHeaderErrorDiagnostic},
-                                                            ClangDiagnosticWidget::InfoBar, {}));
+                                                            ClangDiagnosticWidget::InfoBar, {},
+                                                            "libclang"));
 
         auto widget = new QWidget;
         widget->setLayout(vbox);