When importing a C++ struct, if its owning module requires cplusplus, Swift tried to auto-conform it to certain protocols from the Cxx module. This triggers name lookup in the clang struct, specifically for `__beginUnsafe()` and `__endUnsafe` methods, which imports all of the base structs including their methods.
This moves the import of base structs out of the name lookup request, preventing cycles.
rdar://116426238
