Eliminate Sema::ObjCProtocols. Instead, we place ObjCProtocolDecls in

their own namespace (IDNS_Protocol) and use the normal name-lookup
routines to find them. Aside from the simplification this provides
(one less DenseMap!), it means that protocols will be lazily
deserialized from PCH files.

Make the code size of the selector table block match the code size of
the type and decl blocks.

llvm-svn: 69939