Fix constructor call when reading from AST file.

Pass some default arguments (cribbed from NamespaceDecl).

In doing so, removed the ASTContext& argument from the constructor. It's used by the `operator new` invocation in the factory methods, so it doesn't need to be passed to the constructor.
Silence warning about missing enum value.

Add ObjCNamespace to the kinds of Decls for which we should never be asked to emit LLVM bitcode.