[REG 5.12.3 -> 5.13.0] Excessive qdoc logging breaks builds

When building Qt Creator with documentation, qdoc logs millions of lines like this:

[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/coreplugin/dialogs
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/coreplugin
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/debugger/namedemangler
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/debugger
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/debugger/console
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/coreplugin/actionmanager
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/texteditor/codeassist
[ 9138s] LOG: -I/home/abuild/rpmbuild/BUILD/qt-creator-opensource-src-4.9.0/src/plugins/coreplugin/locator

As a result, the build time is unusuably slow and our build system actually terminates the job due to the massive output. The cause seems to be commit c4d02e5cea9f8657bd74f1f724d048de5c0142b1 ("qdoc: Let qdoc print better log messages").