QDoc: Turn "Unknown base for QML type" warning into report

When processing the documentation for QML types, QDoc attempts to resolve their base type when applicable. In some cases, the base type may be a dependency. However, if so, and the dependency isn't met (as is the case in Qt's CI system which integrates repositories separately), QDoc currently issues a false positive warning. As a consequence, Qt's
documentation configuration contains a spurious regexp that lets QDoc omit/ignore this warning. By changing the warning to a report, we ensure that QDoc will notify the
user of the possible mistake.