cppgc: Fix ConstructionBitProtectsNonAtomicWrites on MSVC

std::atomic loads are marked as nodiscard on MSVC. Fix the warning by
feeding the load into the USE() macro.

Bug: chromium:1056170
Change-Id: I72ca42d42d268c4b961d96618250229a53709472
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2120543Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66866}

test/unittests/heap/cppgc/heap-object-header_unittest.cc

@@ -133,8 +133,8 @@ class ConcurrentGCThread final : public v8::base::Thread {
   void Run() final {
     while (header_->IsInConstruction<HeapObjectHeader::AccessMode::kAtomic>()) {
     }
-    v8::base::AsAtomicPtr(const_cast<size_t*>(&payload_->value))
-        ->load(std::memory_order_relaxed);
+    USE(v8::base::AsAtomicPtr(const_cast<size_t*>(&payload_->value))
+            ->load(std::memory_order_relaxed));
   }
 
  private: