We occasionally have some public `foo.h` header that includes a private
`foo_arch.h` header. Users are expected to include the `foo.h` header
and not the `foo_arch.h`. However, clangd will claim that the `#include`
of `foo.h` is unused if only functions / macros/ types / ... from
`foor_arch.h` is used and nothing from `foo.h`.
This adds the `IWYU pragma: export` comment to the include of
`foo_arch.h` in `foo.h`, so that clangd treats functions / macros /
types provided by `foo_arch.h` as if they were instead provided by
`foo.h`, which fixes the false positives.
