This patch makes the `map_type` and `map_capture_type` arguments of the
`omp.map.info` operation required, which was already an invariant being
verified by its users via `verifyMapClause()`. This makes it clearer, as
getters no longer return misleading `std::optional` values.
Checks for the `mapper_id` argument are moved to a verifier for the
operation, rather than being checked by users.
Functionally NFC, but not marked as such due to a reordering of
arguments in the assembly format of `omp.map.info`.