[REGRESSION] Qt Creator can no longer properly open LLVM

Steps to reproduce:
1. Clone the LLVM repo: https://github.com/llvm/llvm-project.git
2. Open llvm/CMakeLists.txt. I used '-DLLVM_ENABLE_PROJECTS=clang;clang-tools-extra'
when configuring, but I don't know whether it's relevant.
3. Try to locate the file SemanticHighlighting.cpp via the "Files in Current Project" locator (Ctrl-k p).
The file does not show up. In addition, dozens of these messages appear in the console Qt Creator was started from:

SOFT ASSERT [16:25:17.545]: "cmln" in /sda/home/christian/dev/qtcreator/regression-tests/src/src/plugins/cmakeprojectmanager/projecttreehelper.cpp:123
SOFT ASSERT [16:25:17.557]: "cmln" in /sda/home/christian/dev/qtcreator/regression-tests/src/src/plugins/cmakeprojectmanager/projecttreehelper.cpp:141
SOFT ASSERT [16:25:17.557]: "tNode" in /sda/home/christian/dev/qtcreator/regression-tests/src/src/plugins/cmakeprojectmanager/fileapidataextractor.cpp:634

Bisecting shows that the culprit is commit d01c22b8057b5eeea5661777c6dd56a71554bf55, the line numbers from above assertions are for that state.