Add workaround for q++ 8.3.0 (QNX 7.1) compiler by making

sure MaskedPointer is trivially copyable and copy constructible.

Fixes #1758

PiperOrigin-RevId: 679618900
Change-Id: Ia0ebacd8bb43b3fe3b9cd654259bef9072cec46c

absl/flags/internal/flag.h

@@ -373,9 +373,13 @@ class MaskedPointer {
 
   static constexpr int RequiredAlignment() { return 4; }
 
+  constexpr MaskedPointer() : ptr_(nullptr) {}
   constexpr explicit MaskedPointer(ptr_t rhs) : ptr_(rhs) {}
   MaskedPointer(ptr_t rhs, bool is_candidate);
 
+  MaskedPointer(const MaskedPointer& rhs) = default;
+  MaskedPointer& operator=(const MaskedPointer& rhs) = default;
+
   void* Ptr() const {
     return reinterpret_cast<void*>(reinterpret_cast<mask_t>(ptr_) &
                                    kPtrValueMask);