std: fix run tests when symlink is in the rust checkout path
syntax: move callee_id into the expr_ variants
auto merge of #6885 : erickt/rust/move-callee_id, r=catamorphism

The `callee_id` in `ast::expr` in only used in a couple expression variants. This moves the `callee_id` into those branches to make it more clear when its should be used.

Also, it fixes a bug in a std::run test when there is a symlink in the path rust where was checked out.