[libc++] Avoid transitive inclusion for `<__algorithm/find.h>` (#162508)
Currently, `size_t` and `__libcpp_is_constant_evaluated` are obtained by
transitive inclusion in `<__algorithm/find.h>` when `<cwchar>` is not
included. This broke module build when `_LIBCPP_HAS_WIDE_CHARACTERS` is
`1` and caused CI failure. We should explicitly include the
corresponding internal headers.