[Flang][OpenMP] Fix Flang crash and incorrect ordering with OpenMP detached task (#194840) Fixes - [#194563](https://github.com/llvm/llvm-project/issues/194563) This PR fixes the runtime crash and incorrect task ordering reported in the testcase involving: ``` !$omp task if(.false.) depend(out:x) detach(ev) ``` The testcase had two issues: **1. Segmentation fault near omp_fulfill_event(ev)** The detach event handle was not being initialized or preserved correctly before the nested task used it. **2. Incorrect execution order** The task with depend(in:x) was running before the detach event was fulfilled, which violates OpenMP dependency semantics. #### Changes in this PR 1. Treat `parallel master` as a top-level parallel construct in Flang semantics to ensure correct data-sharing behavior. 2. Treat variables used in `detach(...)` as shared so the event handle remains valid across tasks. 3. Fix `OMPIRBuilder` lowering for task `if(.false.)` to use merged-if0 behavior through the normal task runtime path, ensuring correct task allocation, event creation, dependency registration, and proper waiting until `omp_fulfill_event(ev)`. 4. Add an MLIR regression test for `if(false) + depend + detach`.
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