swift234

[ClangImporter] Synthesize the Protocol class if needed.

'Protocol' is the class used by Objective-C to refer to the runtime metadata
for a protocol. It's used by APIs such as NSXPCConnection. The class is
defined as "@interface Protocol : NSObject" in the modern Objective-C runtime.

This is a workaround for a Clang bug, PR19061 / <rdar://problem/16244450>.
We ought to be able to see the definition of Protocol whenever the
ObjectiveC module is loaded, but right now we can't. Once that bug is fixed,
this change should be reverted.

<rdar://problem/16229963>

Swift SVN r14761