libcxx: Mark __temp_value::__temp_value as _LIBCPP_NO_CFI.

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.

Differential Revision: https://reviews.llvm.org/D50743

llvm-svn: 339797