When building for Windows, either on Windows or cross compiling, clang-cl accepts the "-imsvc" (or "/imsvc") flag to tell it where to find Windows header files. E.g.:

../../third_party/llvm-build/Release+Asserts/bin/clang-cl -imsvc../../third_party/depot_tools/win_toolchain/vs_files/8f58c55897a3282ed617055775a77ec3db771b88/win_sdk/Include/10.0.18362.0/um -imsvc../../third_party/depot_tools/win_toolchain/vs_files/8f58c55897a3282ed617055775a77ec3db771b88/win_sdk/Include/10.0.18362.0/shared ...

It doesn't appear that Qt Creator currently handles these flags at all. (`grep -R imsvc` returns no results, and opening the compilation database leads to include errors in the projects source files.)