[Flang][OpenMP] Add LLVM translation support for UNTIED clause in Task

Implementation details:
The UNTIED clause is recognized by setting the flag=0 for the default
case or performing logical OR to flag if other clauses are specified,
and this flag is passed as an argument to the `__kmpc_omp_task_alloc`
runtime call.

Author: Thirumalai-Shaktivel

flang/lib/Lower/OpenMP/OpenMP.cpp

@@ -2844,6 +2844,7 @@ static void genOMP(lower::AbstractConverter &converter, lower::SymMap &symTable,
         !std::holds_alternative<clause::UseDevicePtr>(clause.u) &&
         !std::holds_alternative<clause::InReduction>(clause.u) &&
         !std::holds_alternative<clause::Mergeable>(clause.u) &&
+        !std::holds_alternative<clause::Untied>(clause.u) &&
         !std::holds_alternative<clause::TaskReduction>(clause.u)) {
       TODO(clauseLocation, "OpenMP Block construct clause");
     }

flang/test/Lower/OpenMP/Todo/task_untied.f90

@@ -245,3 +245,16 @@ subroutine task_multiple_clauses()
   !CHECK: omp.terminator
   !$omp end task
 end subroutine task_multiple_clauses
+
+!===============================================================================
+! `untied` clause
+!===============================================================================
+
+!CHECK-LABEL: func.func @_QPomp_task_untied() {
+subroutine omp_task_untied()
+  !CHECK: omp.task untied {
+  !$omp task untied
+    call foo()
+  !CHECK: omp.terminator
+  !$omp end task
+end subroutine omp_task_untied

flang/test/Lower/OpenMP/task.f90

@@ -3003,6 +3003,17 @@ module attributes {omp.is_target_device = true} {
 
 // -----
 
+llvm.func @omp_task_untied() {
+  // The third argument is 0: which signifies the united task
+  // CHECK: {{.*}} = call ptr @__kmpc_omp_task_alloc(ptr @1, i32 %{{.*}}, i32 0, i64 40, i64 0, ptr @{{.*}})
+  omp.task untied {
+    omp.terminator
+  }
+  llvm.return
+}
+
+// -----
+
 llvm.func external @foo_before() -> ()
 llvm.func external @foo() -> ()
 llvm.func external @foo_after() -> ()