rustc_back: Only use archive member filenames

I've been working with some archives generated by MSVC's `lib.exe` tool lately,
and it looks like the embedded name of the members in those archives sometimes
have slahes in the name (e.g. `foo/bar/baz.obj`). Currently the compiler chokes
on these paths as it assumes that each file in the archive is only the filename
(which is what unix does).

This commit interprets the name of each file in all archives as a path and then
only uses the `file_name` portion of the path to extract the file to a separate
location and then reassemble it back into a new archive later. Note that
duplicate filenames are already handled, so this won't introduce any conflicts.

Author: alexcrichton

src/librustc_back/archive.rs

@@ -305,6 +305,8 @@ impl<'a> ArchiveBuilder<'a> {
             };
             if filename.contains(".SYMDEF") { continue }
             if skip(filename) { continue }
+            let filename = Path::new(filename).file_name().unwrap()
+                                              .to_str().unwrap();
 
             // An archive can contain files of the same name multiple times, so
             // we need to be sure to not have them overwrite one another when we