Phase 2 of making the Decl class more lightweight...

Move Identifier/Loc instance variables (and associated getters/setters) down from Decl to ScopedDecl/FieldDecl.

Objc AST's can now inherit from Decl without getting instance variables and types that are C specific. For now, I am keeping NextDeclarator, since I believe it may be useful to ObjC. If not, it can be moved later.

llvm-svn: 41934