[interop] Protocol witnesses for C++ conformances are externally visible & linkable

[hyp]

Describe the bug
If I write a manual conformance for CxxSequence, I get a protocol witness that doesn't have a shared SIL linkage, which leads to this TBD error:

<unknown>:0: error: symbol '$sSo3stdO3__1O046__CxxTemplateInstNSt3__16vectorIjNS_9allocatorG4EEEEV13CreateObjects0B8SequenceA2gHP11endMutating11RawIteratorQzyFTW' (protocol witness for CreateObjects.CxxSequence.endMutating() -> A.RawIterator in conformance __C.std.__1.__CxxTemplateInstNSt3__16vectorIjNS_9allocatorIjEEEE : CreateObjects.CxxSequence in CreateObjects) is in generated IR file, but not in TBD file