QNativeIpcKey: add implicit default ctor

Add an implicitly callable default ctor, to enable default returns,
but still disallow implicit conversion whilst assigning:

  QNativeIpcKey k = QNativeIpcKey::Type::~~      // not allowed
  auto fn = []() -> QNativeIpcKey { return {}; } // allowed

Pick-to: 6.6
Change-Id: I0f8476a588b3931f0e1f41a6c3a3fd8f2eb75b93
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/ipc/qtipccommon.h

@@ -37,7 +37,11 @@ public:
             ;
     static Type legacyDefaultTypeForOs() noexcept;
 
-    explicit constexpr QNativeIpcKey(Type type = DefaultTypeForOs) noexcept
+    constexpr QNativeIpcKey() noexcept
+        : QNativeIpcKey(DefaultTypeForOs)
+    {}
+
+    explicit constexpr QNativeIpcKey(Type type) noexcept
         : d()
     {
         typeAndFlags.type = type;