Always calculate glob map but only for glob uses

Previously calculating glob map was *opt-in*, however it did record
node id -> ident use for every use directive. This aims to see if we
can unconditionally calculate the glob map and not regress performance.

src/librustc_save_analysis/dump_visitor.rs

@@ -1239,7 +1239,6 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> DumpVisitor<'l, 'tcx, 'll, O> {
 
                 // Make a comma-separated list of names of imported modules.
                 let glob_map = &self.save_ctxt.analysis.glob_map;
-                let glob_map = glob_map.as_ref().unwrap();
                 let names = if glob_map.contains_key(&id) {
                     glob_map.get(&id).unwrap().iter().map(|n| n.to_string()).collect()
                 } else {

src/librustc_save_analysis/lib.rs

@@ -1122,8 +1122,6 @@ pub fn process_crate<'l, 'tcx, H: SaveHandler>(
     mut handler: H,
 ) {
     tcx.dep_graph.with_ignore(|| {
-        assert!(analysis.glob_map.is_some());
-
         info!("Dumping crate {}", cratename);
 
         let save_ctxt = SaveContext {