The diff adds Remark to Diagnostic::Level for clang tooling. That makes Remark diagnostic level ready to use in clang-tidy checks: the clang-diagnostic-module-import becomes visible as a part of the change
Test plan:
ninja check-clang ninja check-clang-tools
nit, I would move remark first to have list by increasing severity. It looks like this enum values is not used outside of the sources so I hope it is safe.