Rename Action::ParseRecordBody() to ProcessFieldDecls(), and add a visibility argument.

Remove Action::ObjcAddVisibilityToIvars(). No need for an extra API when it is trivial to add this info to the previous hook.

In general, I want to start migrating away from having Actions prefixed with "Parse" (which is confusing, since the Action API doesn't do any parsing, per se).

llvm-svn: 41973