[flang] This adds the lowering stubs for Open MP.

The lowering bridge will call these lowering hooks to process the Open
MP directives that it iterates over in the PFT.  This is a mock
interface without an implementation in this patch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D80815

flang/include/flang/Lower/AbstractConverter.h

@@ -0,0 +1,19 @@
+//===-- Lower/AbstractConverter.h -------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_LOWER_ABSTRACTCONVERTER_H
+#define FORTRAN_LOWER_ABSTRACTCONVERTER_H
+
+namespace Fortran::lower {
+
+// temporary stub
+class AbstractConverter;
+
+} // namespace Fortran::lower
+
+#endif // FORTRAN_LOWER_ABSTRACTCONVERTER_H

flang/include/flang/Lower/OpenMP.h

@@ -0,0 +1,35 @@
+//===-- Lower/OpenMP.h -- lower Open MP directives --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_LOWER_OPENMP_H
+#define FORTRAN_LOWER_OPENMP_H
+
+namespace Fortran {
+namespace parser {
+struct OpenMPConstruct;
+struct OmpEndLoopDirective;
+} // namespace parser
+
+namespace lower {
+
+class AbstractConverter;
+
+namespace pft {
+struct Evaluation;
+} // namespace pft
+
+void genOpenMPConstruct(AbstractConverter &, pft::Evaluation &,
+                        const parser::OpenMPConstruct &);
+
+void genOpenMPEndLoop(AbstractConverter &, pft::Evaluation &,
+                      const parser::OmpEndLoopDirective &);
+
+} // namespace lower
+} // namespace Fortran
+
+#endif // FORTRAN_LOWER_OPENMP_H

flang/lib/Lower/CMakeLists.txt

@@ -1,5 +1,6 @@
 
 add_flang_library(FortranLower
+  OpenMP.cpp
   PFTBuilder.cpp
 
   LINK_COMPONENTS

flang/lib/Lower/OpenMP.cpp

@@ -0,0 +1,26 @@
+//===-- OpenMP.cpp -- Open MP directive lowering --------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Lower/OpenMP.h"
+#include "flang/Lower/AbstractConverter.h"
+#include "flang/Lower/PFTBuilder.h"
+#include "flang/Parser/parse-tree.h"
+
+#define TODO() llvm_unreachable("not yet implemented")
+
+void Fortran::lower::genOpenMPConstruct(
+    Fortran::lower::AbstractConverter &, Fortran::lower::pft::Evaluation &,
+    const Fortran::parser::OpenMPConstruct &) {
+  TODO();
+}
+
+void Fortran::lower::genOpenMPEndLoop(
+    Fortran::lower::AbstractConverter &, Fortran::lower::pft::Evaluation &,
+    const Fortran::parser::OmpEndLoopDirective &) {
+  TODO();
+}