[Flang][openmp][5.0] Add task_reduction clause.

See OMP-5.0 2.19.5.5 task_reduction Clause.
To add a positive test case we need `taskgroup` directive which is not added hence skipping the test.
This is a dependency for `taskgroup` construct.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D93105

include/llvm/Frontend/OpenMP/OMP.td

@@ -231,6 +231,7 @@ def OMPC_IsDevicePtr : Clause<"is_device_ptr"> {
 }
 def OMPC_TaskReduction : Clause<"task_reduction"> {
   let clangClass = "OMPTaskReductionClause";
+  let flangClassValue = "OmpReductionClause";
 }
 def OMPC_InReduction : Clause<"in_reduction"> {
   let clangClass = "OMPInReductionClause";