CompactStorage: make it default-constructible again

When 30a8e79243084017d23f1c765d5f1cbb86564191 added constructors as a
work-arund for an MSVC issue, the default contructor became deleted.

This hasn't caused trouble so far, as all users apparently pass a
payload object, but I'm about to merge a new user that doesn't, so
this came up.

Fix by bringing the default ctor back using =default.

Amends 30a8e79243084017d23f1c765d5f1cbb86564191.

Pick-to: 6.6
Change-Id: Iaf1a9536c2e0b24a62fdd55c837478a3450da38c
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 1b89d747d7da7bffd515a5521e7dd5212c95f7af)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qfunctionaltools_impl.h

@@ -42,6 +42,7 @@ struct StorageByValue
 template <typename Object, typename Tag = void>
 struct StorageEmptyBaseClassOptimization : Object
 {
+    StorageEmptyBaseClassOptimization() = default;
     StorageEmptyBaseClassOptimization(Object &&o)
         : Object(std::move(o))
     {}