Set debugger when importing tool chains from old settings

Set the default debugger when importing tool chains from old
settings. We just left the field blank before.

Task-number: QTCREATORBUG-4482
Reviewed-by: dt

src/plugins/qt4projectmanager/qtversionmanager.cpp

@@ -185,6 +185,8 @@ QtVersionManager::QtVersionManager()
                 if (tc) {
                     tc->setCompilerPath(fi.absoluteFilePath());
                     tc->setDisplayName(tr("MinGW from %1").arg(version->displayName()));
+                    // The debugger is set later in the autoDetect method of the MinGw tool chain factory
+                    // as the default debuggers are not yet registered.
                     ProjectExplorer::ToolChainManager::instance()->registerToolChain(tc);
                 }
             }