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