[mlir][OpenMP] Pack task private variables into a heap-allocated context struct (#125307)

See RFC:

https://discourse.llvm.org/t/rfc-openmp-supporting-delayed-task-execution-with-firstprivate-variables/83084

The aim here is to ensure that tasks which are not executed for a while
after they are created do not try to reference any data which are now
out of scope. This is done by packing the data referred to by the task
into a heap allocated structure (freed at the end of the task).

I decided to create the task context structure in
OpenMPToLLVMIRTranslation instead of adapting how it is done
CodeExtractor (via OpenMPIRBuilder] because CodeExtractor is (at least
in theory) generic code which could have other unrelated uses.

Author: tblah

flang/lib/Lower/OpenMP/DataSharingProcessor.cpp

@@ -594,7 +594,7 @@ void DataSharingProcessor::doPrivatize(const semantics::Symbol *sym,
           sym, cannotHaveNonDefaultLowerBounds);
       // TODO: currently there are false positives from dead uses of the mold
       // arg
-      if (!result.getInitMoldArg().getUses().empty())
+      if (result.initReadsFromMold())
         mightHaveReadHostSym.insert(sym);
     }
 

mlir/include/mlir/Dialect/OpenMP/OpenMPOps.td

@@ -146,13 +146,24 @@ def PrivateClauseOp : OpenMP_Op<"private", [IsolatedFromAbove, RecipeInterface]>
       return region.empty() ? nullptr : region.getArgument(0);
     }
 
+    /// Returns true if the init region might read from the mold argument
+    bool initReadsFromMold() {
+      BlockArgument moldArg = getInitMoldArg();
+      return moldArg && !moldArg.use_empty();
+    }
+
+    /// Returns true if any region of this privatizer might read from the mold
+    /// argument
+    bool readsFromMold() {
+      return initReadsFromMold() || !getCopyRegion().empty();
+    }
+
     /// needsMap returns true if the value being privatized should additionally
     /// be mapped to the target region using a MapInfoOp. This is most common
     /// when an allocatable is privatized. In such cases, the descriptor is used
     /// in privatization and needs to be mapped on to the device.
     bool needsMap() {
-      BlockArgument moldArg = getInitMoldArg();
-      return moldArg ? !moldArg.use_empty() : false;
+      return initReadsFromMold();
     }
 
     /// Get the type for arguments to nested regions. This should