Core/Utils: Migrate further to Utils::Id

The coreplugin/id.h header is kept for downstream for now.

Change-Id: I8c44590f7b988b3770ecdc177c40783e12353e66
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/generatedfile.cpp

@@ -34,6 +34,8 @@
 #include <QDir>
 #include <QString>
 
+using namespace Utils;
+
 namespace Core {
 
 /*!