mir: improve polymorphic constant bug

This commit changes the span and content of the "collection encountered
polymorphic constant" bug in monomorphization collection to point to the
use of the constant rather than the definition.
Signed-off-by: NDavid Wood <david@davidtw.co>

src/librustc_mir/monomorphize/collector.rs

@@ -627,8 +627,9 @@ fn visit_const(&mut self, constant: &&'tcx ty::Const<'tcx>, location: Location)
                     Ok(val) => collect_const_value(self.tcx, val, self.output),
                     Err(ErrorHandled::Reported(ErrorReported) | ErrorHandled::Linted) => {}
                     Err(ErrorHandled::TooGeneric) => span_bug!(
-                        self.tcx.def_span(def.did),
-                        "collection encountered polymorphic constant",
+                        self.body.source_info(location).span,
+                        "collection encountered polymorphic constant: {}",
+                        substituted_constant
                     ),
                 }
             }