Rollup merge of rust-lang#49312 - GuillaumeGomez:fix-ie11-search, r=QuietMisdreavus

Fix IE11 search

Fixes rust-lang#49263.

r? @QuietMisdreavus

I got this code from: https://stackoverflow.com/a/30867255/7879548

Author: kennytm

src/librustdoc/html/static/main.js

@@ -49,6 +49,13 @@
 
     var themesWidth = null;
 
+    if (!String.prototype.startsWith) {
+        String.prototype.startsWith = function(searchString, position) {
+            position = position || 0;
+            return this.indexOf(searchString, position) === position;
+        };
+    }
+
     function hasClass(elem, className) {
         if (elem && className && elem.className) {
             var elemClass = elem.className;