I don't know.
This recent post seems relevant: Is there a way to list the APIs provided by my dynamic library?. It seems to suggest that the emitted header contains Obj-C-specific features. (The OP speaks of -emit-objc-header-path, not -emit-clang-header-path, but in my testing both flags emit the same C header, at least when C++ interop is not enabled. I haven't tried it with C++ interop.)