Bug#1119091: Bug #1119091 : qt6-tools: build with default clang/llvm

[Jeremy Bícha]

In case you want to cherry-pick the llvm 21 enablement commits before
updating to 6.9.3, they can be found at
https://launchpad.net/ubuntu/+source/qt6-tools/6.9.2-2ubuntu1

Thank you,
Jeremy Bícha