[clang] CodeGen: Make getOrEmitProtocol public for Swift

Summary:
Swift would like to use clang's apis to emit protocol declarations.

This commits adds the public API:

```
 emitObjCProtocolObject(CodeGenModule &CGM, const ObjCProtocolDecl *p);
```

rdar://60888524

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77077