I set up a custom compiler in Qt creator under "Build & Run"/"Compilers". I set the path to "/usr/bin/clang++-5.0". Then I set my only kit to use that compiler. Then I created a new project from the plain c++ app template and set it to use qmake.
Running qmake on that project results in the following error:
14:01:44: Running steps for project test2...
14:01:44: Starting: "/home/pmitas/Qt/5.10.0/gcc_64/bin/qmake" /home/pmitas/qtcreator-projects/test2/test2.pro -spec linux-clang CONFIG+=debug CONFIG+=qml_debug
Project ERROR: Cannot run compiler 'clang++'. Maybe you forgot to setup the environment?
14:01:44: The process "/home/pmitas/Qt/5.10.0/gcc_64/bin/qmake" exited with code 3.
Error while building/deploying project test2 (kit: Desktop Qt 5.10.0 GCC 64bit)
When executing step "qmake"
14:01:44: Elapsed time: 00:00.