rustc: Use the "real" realpath function

The logic of the custom realpath function in metadata::loader was incorrect, but
the logic in util::fs was correct.

Closes #13890
auto merge of #13903 : alexcrichton/rust/issue-13890, r=thestinger

The logic of the custom realpath function in metadata::loader was incorrect, but
the logic in util::fs was correct.

Closes #13890