[Reflection] Fix async task slab size calculation.

The calculation failed to account for padding after the slab struct itself. We already account for this padding in HeaderSize, so use that instead of the raw struct size.

rdar://87607280

Author: mikeash

include/swift/Reflection/ReflectionContext.h

@@ -1372,8 +1372,8 @@ class ReflectionContext
     Chunk.Length = Slab->CurrentOffset;
     Chunk.Kind = AsyncTaskAllocationChunk::ChunkKind::Unknown;
 
-    // Total slab size is the slab's capacity plus the slab struct itself.
-    StoredPointer SlabSize = Slab->Capacity + sizeof(*Slab);
+    // Total slab size is the slab's capacity plus the header.
+    StoredPointer SlabSize = Slab->Capacity + HeaderSize;
 
     return {llvm::None, {Slab->Next, SlabSize, {Chunk}}};
   }