[cxxmodules] Invert the assert condition.
[cxxmodules] Invert the assert condition.

Fixes the builds with -Druntime_cxxmodules=On on OSX.

root-mirror/root,karies/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root,karies/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,karies/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,root-mirror/root