Cache QUrl::idnWhiteList() absent user_idn_whitelist

Instead of creating a QStringList from a static array anew for each
call, cache the result in a static variable. If we have to pay for the
overhead of implicitly-shared classes, we should at least reap the
benefits, too.

Use IILE to gain automatic thread-safety (thread-safe statics).

Change-Id: Ib92dd9cb85f84e013f98ca81565cc392bb39e76b
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/io/qurlidna.cpp

@@ -2605,13 +2605,16 @@ QStringList QUrl::idnWhitelist()
 {
     if (user_idn_whitelist)
         return *user_idn_whitelist;
-    QStringList list;
-    list.reserve(idn_whitelist_size);
-    unsigned int i = 0;
-    while (i < idn_whitelist_size) {
-        list << QLatin1String(idn_whitelist[i]);
-        ++i;
-    }
+    static const QStringList list = [] {
+        QStringList list;
+        list.reserve(idn_whitelist_size);
+        unsigned int i = 0;
+        while (i < idn_whitelist_size) {
+            list << QLatin1String(idn_whitelist[i]);
+            ++i;
+        }
+        return list;
+    }();
     return list;
 }