totally_ordered_wrapper: make default-constructible

Pointers are.

The default-constructor is = default'ed, meaning it has the same
semantics as the raw pointer it replaces:

   totally_ordered_wrapper<int*> p; // partially-formed (uninit'ed)
   totally_ordered_wrapper<int*> p{}; // well-formed (nullptr)

   totally_ordered_wrapper<int*> a[1024]; // doesn't write 8KiB of NULs

Change-Id: I0d2e906bc5ec45d95b03a3af451167fec84439fd
Reviewed-by: Rym Bouabid <rym.bouabid@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>

src/corelib/global/qcomparehelpers.h

@@ -619,6 +619,7 @@ class totally_ordered_wrapper
 
     P ptr;
 public:
+    totally_ordered_wrapper() noexcept = default;
     explicit constexpr totally_ordered_wrapper(P p) noexcept : ptr(p) {}
 
     constexpr P get() const noexcept { return ptr; }