I was able to run a debug configuration built with clang-cl (latest clang that comes with VS-2019) with a custom QT built with VS-2017 without any issues.
If I try to build my QT app using the Win64 Release configuration (against same QT VS-2017, Release), I get on the final link stage:
lld-link : error : unknown file type: .obj\release\qaxobject.obj
Looking on the web I see that clang-cl tries to be binary compatible with Visual Studio and I find strange I only get this issue on the Release build. Also not clear to me from QT project if QT can be built with latest clang-cl on Windows (if there is indeed an ABI problem on Release)