Fix categorization of upvars of capture-by-reference unboxed closures

In particular, this causes mutation of an upvar to correctly mark
it as mutable during adjustment.  This makes borrowck correctly
flag conflicting borrows, etc.

We still seem to generate incorrect code in trans which copies the upvar
by value into the closure.  This remains to be fixed.

src/librustc/middle/mem_categorization.rs

@@ -604,17 +604,21 @@ pub fn cat_def(&self,
                           ty::FnMutUnboxedClosureKind => ast::Many,
                           ty::FnOnceUnboxedClosureKind => ast::Once,
                       };
-                      Ok(Rc::new(cmt_ {
-                          id: id,
-                          span: span,
-                          cat: cat_copied_upvar(CopiedUpvar {
-                              upvar_id: var_id,
-                              onceness: onceness,
-                              capturing_proc: fn_node_id,
-                          }),
-                          mutbl: MutabilityCategory::from_local(self.tcx(), var_id),
-                          ty: expr_ty
-                      }))
+                      if self.typer.capture_mode(fn_node_id) == ast::CaptureByRef {
+                          self.cat_upvar(id, span, var_id, fn_node_id)
+                      } else {
+                          Ok(Rc::new(cmt_ {
+                              id: id,
+                              span: span,
+                              cat: cat_copied_upvar(CopiedUpvar {
+                                  upvar_id: var_id,
+                                  onceness: onceness,
+                                  capturing_proc: fn_node_id,
+                              }),
+                              mutbl: MutabilityCategory::from_local(self.tcx(), var_id),
+                              ty: expr_ty
+                          }))
+                      }
                   }
                   _ => {
                       self.tcx().sess.span_bug(