To map `fir.boxchar` types reliably onto an offload target, such as a
GPU, the `omp.map.info` operation is used to map the underlying data
pointer (`fir.ref<fir.char<k, ?>>`) wrapped by the `fir.boxchar` MLIR
value. The `omp.map.info` operation needs a pointer to the underlying
data pointer.
Given a reference to a descriptor (`fir.box`), the `fir.box_offset` is
used to obtain the address of the underlying data pointer. This PR
extends `fir.box_offset` to provide the same functionality for
`fir.boxchar` as well.
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