Fix CFI issues in <future>
This change fixes errors reported by Control Flow Integrity (CFI) checking when using std::packaged_task. The errors mostly stem from casting the underlying storage (__buf_) to __base*, even if it is uninitialized. The solution is to wrap __base* access to __buf_ behind a getter marked with _LIBCPP_NO_CFI.
Differential Revision: https://reviews.llvm.org/D82627