- only complain about duplicate 'sources' lines if they contain
more than 1 elements each (before, 0 and 1 element lists were
counted too). In practice, this only has an effect for
clang/lib/Headers/BUILD.gn
- make the '# NOSORT' diag actually work. This too only affects
clang/lib/Headers/BUILD.gn.
In aggregate, changes to clang/lib/Headers/BUILD.gn still can't
be auto-merged, but for a slighly better reason.