Clang code model can't find clang headers when built with cmake

If qtcreator 4.14 is built with cmake (against a system clang) the clang code model can't find the clang headers in projects that use a kit with the clang compiler, giving errors such as

Warning: The code model could not parse an include file
...
wchar.h:35:10: error: 'stddef.h' file not found

This doesn't happen if I build it with qmake. Looking at the output of 'strings /usr/lib/qtcreator/plugins/libClangCodeModel.so', when built with qmake I see the path

/usr/lib/clang/11.0.0/include

which is where the headers are located. When built with cmake, I see

lib/qtcreator/clang/lib/clang/11.0.0/include

which is wrong.