adding debug in consume_body function

When in debug_assertions=true mode, the function consume_body lacks some debug output, which makes it harder to follow the control flow. This commit adds this needed debug.

src/librustc/middle/expr_use_visitor.rs

@@ -288,6 +288,8 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
     }
 
     pub fn consume_body(&mut self, body: &hir::Body) {
+        debug!("consume_body(body={:?})", body);
+        
         for arg in &body.arguments {
             let arg_ty = return_if_err!(self.mc.infcx.node_ty(arg.pat.id));