Fix Modal.confirm content layout

close #9374

created a BFC to resolve multiple lines of title render bug

https://www.smashingmagazine.com/2017/12/understanding-css-layout-block-formatting-context/

components/modal/style/confirm.less

@@ -25,6 +25,10 @@
       font-weight: 500;
       font-size: @font-size-lg;
       line-height: 22px;
+      display: block;
+      // create BFC to avoid
+      // https://user-images.githubusercontent.com/507615/37702510-ba844e06-2d2d-11e8-9b67-8e19be57f445.png
+      overflow: auto;
     }
 
     .@{confirm-prefix-cls}-content {
@@ -38,7 +42,6 @@
       font-size: 22px;
       margin-right: 16px;
       float: left;
-      min-height: 48px;
     }
   }