[flang][OpenMP] Added support for lowering OpenMP taskyield construct

[SouraVX]

Summary

This patch lower !OMP TASKYIELD construct from PFT to
OpenMPDialect operations.
Construct is lowered with conformance to OpenMP 5.0 spec.

Patch is carved out of following approved PR:
https://github.com/flang-compiler/f18-llvm-project/pull/297

[kiranchandramohan]

LGTM.

flang/lib/Lower/OpenMP.cpp

case llvm::omp::Directive::OMPD_barrier:
     absConv.getFirOpBuilder().create<mlir::omp::BarrierOp>(
         absConv.getCurrentLocation());
     break;
   case llvm::omp::Directive::OMPD_taskwait:
     absConv.getFirOpBuilder().create<mlir::omp::TaskwaitOp>(
         absConv.getCurrentLocation());
     break;
   case llvm::omp::Directive::OMPD_taskyield:
-    TODO();
+    absConv.getFirOpBuilder().create<mlir::omp::TaskyieldOp>(
+        absConv.getCurrentLocation());
+    break;
   case llvm::omp::Directive::OMPD_target_enter_data:
     TODO();
   case llvm::omp::Directive::OMPD_target_exit_data:
     TODO();
   case llvm::omp::Directive::OMPD_target_update:
     TODO();
   case llvm::omp::Directive::OMPD_ordered:
     TODO();

flang/unittests/Lower/OpenMPLoweringTest.cpp

TEST_F(OpenMPLoweringTest, TaskWait) {
   EXPECT_EQ(taskWaitDirective.v, llvm::omp::Directive::OMPD_taskwait);
   auto taskWaitOp = mlirOpBuilder->create<mlir::omp::TaskwaitOp>(
       mlirOpBuilder->getUnknownLoc());
 
   EXPECT_EQ(taskWaitOp.getOperationName(), "omp.taskwait");
   EXPECT_EQ(succeeded(taskWaitOp.verify()), true);
 }
 
+TEST_F(OpenMPLoweringTest, TaskYield) {
+  // Construct a dummy parse tree node for `!OMP taskyield`.
+  struct Fortran::parser::OmpSimpleStandaloneDirective taskYieldDirective(
+      llvm::omp::Directive::OMPD_taskyield);
+
+  // Check and lower the `!OMP taskyield` node to `TaskYieldOp` operation of
+  // OpenMPDialect.
+  EXPECT_EQ(taskYieldDirective.v, llvm::omp::Directive::OMPD_taskyield);
+  auto taskYieldOp = mlirOpBuilder->create<mlir::omp::TaskyieldOp>(
+      mlirOpBuilder->getUnknownLoc());
+
+  EXPECT_EQ(taskYieldOp.getOperationName(), "omp.taskyield");
+  EXPECT_EQ(succeeded(taskYieldOp.verify()), true);
+}
+
 // main() from gtest_main