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

[SouraVX]

Summary

This patch lower !OMP TASKWAIT construct from PFT to
OpenMPDialect operations.
Construct is lowered with conformance to OpenMP 4.5 spec.

Patch is carved out of following approved PR:
https://github.com/flang-compiler/f18-llvm-project/pull/280

[kiranchandramohan]

LGTM. Please wait for a day.

flang/lib/Lower/OpenMP.cpp

static void genOMP(Fortran::lower::AbstractConverter &absConv,
   switch (directive.v) {
   default:
     break;
   case llvm::omp::Directive::OMPD_barrier:
     absConv.getFirOpBuilder().create<mlir::omp::BarrierOp>(
         absConv.getCurrentLocation());
     break;
   case llvm::omp::Directive::OMPD_taskwait:
-    TODO();
+    absConv.getFirOpBuilder().create<mlir::omp::TaskwaitOp>(
+        absConv.getCurrentLocation());
+    break;
   case llvm::omp::Directive::OMPD_taskyield:
     TODO();
   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();

flang/unittests/Lower/OpenMPLoweringTest.cpp

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