[Windows] Cloned kit loses the compiler when Qt Creator is restarted

When the compiler of a cloned kit is changed, the compiler setting is lost i.e. it is <no compiler> when Qt Creator is restarted. The compiler needs to be set again and the kit works fine - until you need to restart the Qt Creator again. If the cloned kit has the same compiler as the original, the problem is not reproducible.

We were able to reproduce this with LLVM 5.0 32 bit for Windows when cloning Desktop Qt 5.9.2 MSVC2015

32bit, but it doesn't seem to be tied to any setup. Steps to reproduce:
1. In Build and Run -> Kits: clone auto-detected kit
2. In the cloned kit, enter "win32-clang-msvc" in Qt-mkspec, select <No compiler> for C and "LLVM 32bit based on MSVC2015" for C++
3. Close Qt Creator and restart it
4. The kit has <No compiler> for C++ compiler

This seems to be regression and doesn't seem to happen on mac, but I didn't test all the possible combinations.