[libc++] Use _LIBCPP_NO_UNIQUE_ADDRESS for base in join_view

Despite the comment saying `[[no_unique_address]]` on the `__base_` data member
makes clang crash, this does not seem to be true on CI. So, mark `__base_` with
`_LIBCPP_NO_UNIQUE_ADDRESS`.

Differential Revision: https://reviews.llvm.org/D119208