Since befaa35212dbaac39cd76a8dc748e4df7c90a0d9, the CI stably failed for
the generic-no-wide-characters build, because in no-`wchar_t` modes, the
header for `__remove_cv_t` wasn't properly included.
This PR adds the missing include of `<__type_traits/remove_cv.h>`.
As drive-by, `<__cstddef/size_t.h>` and
`<__type_traits/is_constant_evaluated.h>`, which are included by
`<cwchar>`, are also made included by `<string>` to avoid potential
regression as we're using `size_t` and
`__libcpp_is_constant_evaluated()` in `<string>`.
26ee552ef5