[flang] Add CALL FLUSH(n) legacy extension

[klausler]

Summary

Prior to the introduction of the FLUSH statement in Fortran 2003,
implementations provided a FLUSH subroutine.

We can't yet put Fortran code into the runtime, so this subroutine
is in C++ with a Fortran-mangled entry point name.

[klausler]

Update: get the name mangling right.

[klausler]

Pass argument by reference, of course.

[pmccormick]

LGTM -- thanks for the quick turnaround.

flang/include/flang/Runtime/extensions.h

+//===-- include/flang/Runtime/extensions.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
+//
+//===----------------------------------------------------------------------===//
+
+// These C-coded entry points with Fortran-mangled names implement legacy
+// extensions that will eventually be implemented in Fortran.
+
+#ifndef FORTRAN_RUNTIME_EXTENSIONS_H_
+#define FORTRAN_RUNTIME_EXTENSIONS_H_
+
+#define FORTRAN_SUBROUTINE_NAME(name) name##_
+
+extern "C" {
+
+// CALL FLUSH(n) antedates the Fortran 2003 FLUSH statement.
+void FORTRAN_SUBROUTINE_NAME(flush)(const int &unit);
+
+} // extern "C"
+#endif // FORTRAN_RUNTIME_EXTENSIONS_H_

flang/runtime/CMakeLists.txt

add_flang_library(FortranRuntime
   derived.cpp
   derived-api.cpp
   descriptor.cpp
   descriptor-io.cpp
   dot-product.cpp
   edit-input.cpp
   edit-output.cpp
   environment.cpp
+  extensions.cpp
   extrema.cpp
   file.cpp
   findloc.cpp
   format.cpp
   internal-unit.cpp
   iostat.cpp
   io-api.cpp
   io-error.cpp

flang/runtime/extensions.cpp

+//===-- runtime/extensions.cpp --------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// These C-coded entry points with Fortran-mangled names implement legacy
+// extensions that will eventually be implemented in Fortran.
+
+#include "flang/Runtime/extensions.h"
+#include "flang/Runtime/io-api.h"
+
+extern "C" {
+
+// SUBROUTINE FLUSH(N)
+//   FLUSH N
+// END
+namespace Fortran::runtime::io {
+void FORTRAN_SUBROUTINE_NAME(flush)(const int &unit) {
+  Cookie cookie{IONAME(BeginFlush)(unit, __FILE__, __LINE__)};
+  IONAME(EndIoStatement)(cookie);
+}
+} // namespace Fortran::runtime::io
+} // extern "C"