...and "So" for the Swift-side representation of an Objective-C-side
conformance. These manglings are never used in public symbols, but
they're stil good to be consistent about.
They need external linkage not linkonce_odr which can only be used by defintions
not declarations.
This got exposed by clang imported protocol witness table declarations.
They get assigned shared linkage since they are potentially not unique.
rdar://26563441