Add support for nounused --extern flag

This adds `nounused` to the set of extern flags:
`--extern nounused:core=/path/to/core/libcore.rlib`.

The effect of this flag is to suppress `unused-crate-dependencies`
warnings relating to the crate.

compiler/rustc_interface/src/tests.rs

@@ -66,6 +66,7 @@ where
         location: ExternLocation::ExactPaths(locations),
         is_private_dep: false,
         add_prelude: true,
+        nounused_dep: false,
     }
 }