libcxx: Disable CFI in function std::get_temporary_buffer.

[pcc]

Summary

The specification of this function mandates a cast to uninitialized
T*, which is forbidden under CFI.

[mclow.lists]

I think that there needs to be more docs on the CFI checking; https://clang.llvm.org/docs/ControlFlowIntegrity.html is about using it, not about the design, goals, and implementation.

That being said, this looks fine to me.

libcxx/trunk/include/memory

#endif
     _LIBCPP_INLINE_VISIBILITY raw_storage_iterator  operator++(int)
         {raw_storage_iterator __t(*this); ++__x_; return __t;}
 #if _LIBCPP_STD_VER >= 14
     _LIBCPP_INLINE_VISIBILITY _OutputIterator base() const { return __x_; }
 #endif
 };
 
 template <class _Tp>
+_LIBCPP_NO_CFI
 pair<_Tp*, ptrdiff_t>
 get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
 {
     pair<_Tp*, ptrdiff_t> __r(0, 0);
     const ptrdiff_t __m = (~ptrdiff_t(0) ^
                            ptrdiff_t(ptrdiff_t(1) << (sizeof(ptrdiff_t) * __CHAR_BIT__ - 1)))
                            / sizeof(_Tp);
     if (__n > __m)