Add support for Qt clang builds on Windows

A lot of work has gone into making Qt compile on windows with clang. This work is coordinated via QTBUG-50804.

The only remaining module that does not compile yet is WebEngine and it turns itself off from such Windows builds. There is a task for it already (QTBUG-66664) too.

The problem is that the build keeps breaking because the CI does not check new code coming in. The task is to add a test configuration to the CI to ensure permanent support.