[libc++] Temporarily not use compiler intrinsics for some type traits in Objective-C++ mode.

Currently, there are bugs in Clang's intrinsics for type traits when
handling Objective-C++ `id` (e.g. in `add_pointer`). As a temporary
workaround, don't use these intrinsics in the Objective-C++ mode.

Differential Revision: https://reviews.llvm.org/D145186

(cherry picked from commit 87cf39aa)