Migrate from GCC MinGW to LLVM-MinGW

We have the LLVM-MinGW as a CI test configuration for a while now.

Migrating to LLVM-MinGW would bring the Windows Arm64 support for MinGW, which the GCC package doesn't offer.

Other benefits would be:

• faster toolchain, thus happier customers (GCC MinGW on Windows is pretty slow, the linker is slow, the debugger is slow etc).
• potential support for UWP APIs. Clang does have clang-cl support, which means that the compiler is able to handle MSVC quirks.
  See MinGW GCC support · Issue #1037 · microsoft/cppwinrt (github.com) 
  This would allow Qt Bluetooth to be compiled with the new APIs in the same way as the MSVC version
• Qt Webengine build. Similar story to the one above. clang-cl can compile Qt Webengine, a LLVM-MinGW version should also be able to handle this.
• Faster debugging
• Cross-compilation of Windows applications from macOS os Linux hosts

I've opened up a ticket for WinRT for LLVM-MinGW at Windows Runtime (WinRT) APIs support · Issue #307 · mstorsjo/llvm-mingw (github.com)