libcxx: Mark __temp_value::__temp_value as _LIBCPP_NO_CFI.

[pcc]

Summary

This constructor needs to cast a pointer to uninitialized
memory to a pointer to object type in order to call
allocator_traits::construct(). This cast is not allowed when CFI cast
checks are enabled.

I did this instead of marking __addr() as _LIBCPP_NO_CFI so that we
don't lose CFI checks on get() or the dtor.

[mclow.lists]

LGTM.

include/memory

struct __temp_value {
 
     typename aligned_storage<sizeof(_Tp), alignof(_Tp)>::type __v;
     _Alloc &__a;
 
     _Tp *__addr() { return reinterpret_cast<_Tp *>(addressof(__v)); }
     _Tp &   get() { return *__addr(); }
 
     template<class... _Args>
-    __temp_value(_Alloc &__alloc, _Args&& ... __args) : __a(__alloc)
-    { _Traits::construct(__a, __addr(), _VSTD::forward<_Args>(__args)...); }
+    _LIBCPP_NO_CFI
+    __temp_value(_Alloc &__alloc, _Args&& ... __args) : __a(__alloc) {
+      _Traits::construct(__a, reinterpret_cast<_Tp*>(addressof(__v)),
+                         _VSTD::forward<_Args>(__args)...);
+    }
 
     ~__temp_value() { _Traits::destroy(__a, __addr()); }
     };
 #endif
 
 template<typename _Alloc, typename = void, typename = void>
 struct __is_allocator : false_type {};