QtCreator doesn't pass mkspec compiler parameters to clangd compile_commands.json

Since QtCreator is using clangd for the code model we always had a huge count of false errors in our project when having our ARM target Kit selected. Today I found out that this was caused by QtCreator not passing the parameters of the specified mkspec file to the generated compile_commands.json file.

This causes QtCreator to include different files than the clangd code model expects, which causes a lot of false error messages. See attached errors.png. It also generates a lot of "cannot initialize object parameter of type ... with expression of type ..." errors like seen in https://bugreports.qt.io/browse/QTCREATORBUG-28016. But those I could not reproduce in the sample project and I can't share the code of our real project.

When adding the parameters in the .pro file through "QMAKE_CXXFLAGS +=", they are added to the compile_commands.json file (see arguments.png) and the code model works perfectly fine.

In our case the problem was, that our target is hard-floating, but since the parameter "-mfloat-abi=hard", which is defined in our mkspec linux-oe-g++ was not passed into the compile_commands.json it did not work correctly.

I think QtCreator should pass all CXXFlags which are effective for compile also to the clangd compile_commands.json file.