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

@@ -2605,6 +2605,7 @@ QStringList QUrl::idnWhitelist()
 {
     if (user_idn_whitelist)
         return *user_idn_whitelist;
+    static const QStringList list = [] {
         QStringList list;
         list.reserve(idn_whitelist_size);
         unsigned int i = 0;
@@ -2613,6 +2614,8 @@ QStringList QUrl::idnWhitelist()
             ++i;
         }
         return list;
+    }();
+    return list;
 }
 
 /*!