C++ IR: Add access specifiers to fields and populate in AstVisitor.

The access specifiers aren't being serialized to JSON yet, so no need to change
the JSON side (that will happen in a followup CL).

PiperOrigin-RevId: 397254542

rs_bindings_from_cc/ir.h

diff --git a/rs_bindings_from_cc/ir.h b/rs_bindings_from_cc/ir.h
index fe958d0..89d05c3 100644
--- a/rs_bindings_from_cc/ir.h
+++ b/rs_bindings_from_cc/ir.h
@@ -118,12 +118,20 @@
   bool is_inline;
 };
 
+// Access specifier for a member or base class.
+enum AccessSpecifier {
+  kPublic,
+  kProtected,
+  kPrivate,
+};
+
 // A field (non-static member variable) of a record.
 struct Field {
   nlohmann::json ToJson() const;
 
   Identifier identifier;
   Type type;
+  AccessSpecifier access;
 };
 
 // A record (struct, class, union).