[flang][OpenMP] Lower target in_reduction (#199967)
This patch carries `in_reduction` through `omp.target` for host
execution.
The lowering looks up the task-reduction private storage with
`__kmpc_task_reduction_get_th_data` and binds the target region argument
to that private pointer. This makes uses inside the target region refer
to the task-private reduction storage instead of continuing to use the
original variable.
This also fixes the `omp::TargetOp::build(TargetOperands)` path so
`in_reduction` operands are preserved instead of being dropped.
On the Flang side, `target in_reduction` list items are added to the
target map entries when needed, giving the translation a matching mapped
value to rewrite.
### Stack / review order
This PR is related to the OpenMP task-reduction translation stack:
1. #199565 — `omp.taskgroup task_reduction` translation
2. #199670 — `omp.taskloop` `reduction` / `in_reduction` translation
3. #202611 — explicit `omp.task in_reduction` translation
This PR is a target-specific sibling follow-up:
* #199967 — `omp.target in_reduction` host lowering
The intended main review order for the task/taskloop stack is:
#199565 → #199670 → #202611
This PR should be reviewed as the `omp.target in_reduction` host-side
lowering work built on the same task-reduction runtime model, not as a
dependency of #202611.
Addresses #199904.
Assisted-by: Claude Opus 4.8 and ChatGPT 5.5
