Clang mode model has issues with namespaced Qt builds

As suspected in QTCREATORBUG-21765, the clang code model has issues resolving Qt types when a namespaced Qt build is used.

How to reproduce:

1. Setup a kit with a namespaced Qt 5.12
2. Enable the clang code model
3. Open e.g. the qtcreator project
4. Open any source file, e.g. main.cpp in the screenshot
5. Observe the countless clang diagnostics about undeclared/incomplete types and undeclared templates.
6. Switch to a kit of the same Qt version without namespace
7. Observe the diagnostics disappear