Fix test.

Author: cjgillot

src/test/run-make-fulldeps/hotplug_codegen_backend/the_backend.rs

@@ -26,7 +26,6 @@ use rustc_span::symbol::Symbol;
 use rustc_target::spec::Target;
 use std::any::Any;
 use std::path::Path;
-use std::sync::Arc;
 
 pub struct NoLlvmMetadataLoader;
 
@@ -57,7 +56,7 @@ impl CodegenBackend for TheBackend {
             tcx.arena.alloc(Default::default()) // Just a dummy
         };
         providers.is_reachable_non_generic = |_tcx, _defid| true;
-        providers.exported_symbols = |_tcx, _crate| Arc::new(Vec::new());
+        providers.exported_symbols = |_tcx, _crate| &[];
     }
 
     fn provide_extern(&self, providers: &mut Providers) {