Fix leading comments added from previous node

This fixes an issue that comments were added as leading comments
to expressions, which are not immediately folowing the comment.
This was also reported and fixed in espree, and is basically a port to
babylon. eslint/espree#256

this also fixes an issue in babel-eslint with certain rules babel/babel-eslint#289

test/fixtures/jsx/basic/10/expected.json

@@ -117,27 +117,8 @@
                   "column": 31
                 }
               },
-              "name": "a",
-              "leadingComments": null
-            },
-            "leadingComments": [
-              {
-                "type": "CommentBlock",
-                "value": " this is a comment ",
-                "start": 4,
-                "end": 27,
-                "loc": {
-                  "start": {
-                    "line": 1,
-                    "column": 4
-                  },
-                  "end": {
-                    "line": 1,
-                    "column": 27
-                  }
-                }
-              }
-            ]
+              "name": "a"
+            }
           },
           "children": [
             {
@@ -220,4 +201,4 @@
       }
     }
   ]
-}
+}