Cannot compile a file in a non-utf8 directory

```
$ mkdir $(echo 'foo\xff') 
$ touch $(echo 'foo\xff')/foo.rs
$ rustc $(echo 'foo\xff')/foo.rs
error: couldn't read foo�/foo.rs: couldn't open file (no such file or directory (No such file or directory); path=foo�/foo.rs; mode=open; access=read)
```
