for Objective-C protocols, including:
- Using the first declaration as the canonical declaration
- Using the definition as the primary DeclContext
- Making sure that all declarations have a pointer to the definition
data, and that we know which declaration is the definition
- Serialization support for redeclaration chains and for adding
definitions to already-serialized declarations.
However, note that we're not taking advantage of much of this code
yet, because we're still re-using ObjCProtocolDecls.
llvm-svn: 147410