I don't think the issue is that Swift doesn't know how to handle them; Clang is the thing that handles C/C++ headers for us.
Apparently @scanon implemented support for the suffix as a Microsoft extension at one point, but it was removed because the Clang maintainers didn't think it was an actual extension.
I've not had time to go back around and try to figure out what the reviewers are looking for, but wouldn't mind someone else digging into the modules behaviour to get that resolved.
The one several lines above never causes an error. So I don't think it's going to be a module problem.
Though at the end of the build I do get cxx-interop shim errors, so it wasn't technically a successful build.
I'll drop the sdk into Finder tomorrow and let it index everything and do a deeper search.
Something must be expanding them, and it sounds like that's the proof the reviewers want to see.