Fix UninitializedMemoryHacks.h for updated libc++ (std::__compressed_pair removed) (#2403)

Summary:
Update std_vector_layout, as __compressed_pair was removed in libc++ in llvm/llvm-project#76756

Fixes: #2351

Pull Request resolved: #2403

Reviewed By: Gownta

Differential Revision: D72390157

Pulled By: yfeldblum

fbshipit-source-id: ae25ff840d7ad6e8ea352895f59c2eea581a9fcf

Author: pps83

folly/memory/UninitializedMemoryHacks.h

@@ -309,7 +309,11 @@ struct std_vector_layout {
 
   pointer __begin_;
   pointer __end_;
+#ifdef _LIBCPP_COMPRESSED_PAIR
+  _LIBCPP_COMPRESSED_PAIR(pointer, __cap_ = nullptr, allocator_type, __alloc_);
+#else
   std::__compressed_pair<pointer, allocator_type> __end_cap_;
+#endif
 };
 
 template <typename T>