emit less info when we warn

Actually, I meant to make this use `delay_span_bug`

Author: nikomatsakis

src/librustc/infer/error_reporting/mod.rs

@@ -269,14 +269,13 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     RegionResolutionError::GenericBoundFailure(ref origin, ..) => {
                         self.tcx.sess.span_warn(
                             origin.span(),
-                            &format!("not reporting region error due to -Znll: {:?}",
-                                     error));
+                            "not reporting region error due to -Znll");
                     }
 
                     RegionResolutionError::SubSupConflict(ref rvo, ..) => {
                         self.tcx.sess.span_warn(
                             rvo.span(),
-                            &format!("not reporting region error due to -Znll: {:?}", error));
+                            "not reporting region error due to -Znll");
                     }
                 }
             }

src/test/ui/nll/named-region-basic.stderr

@@ -1,4 +1,4 @@
-warning: not reporting region error due to -Znll: SubSupConflict(AddrOfRegion($DIR/named-region-basic.rs:19:5: 19:8), Reborrow($DIR/named-region-basic.rs:19:5: 19:8), ReFree(DefId { krate: CrateNum(0), index: DefIndex(0:3) => named_region_basic[317d]::foo[0] }, BrNamed(CrateNum(0):DefIndex(1:10), 'b(88))), Reborrow($DIR/named-region-basic.rs:19:5: 19:8), ReFree(DefId { krate: CrateNum(0), index: DefIndex(0:3) => named_region_basic[317d]::foo[0] }, BrNamed(CrateNum(0):DefIndex(1:9), 'a(86))))
+warning: not reporting region error due to -Znll
   --> $DIR/named-region-basic.rs:19:5
    |
 19 |     &*x