[NFC][flang] Introduce new helper TODO macro for further development

[schweitz]

Summary

The Todo.h header file allows flang developers to mark parts of the (lowering) project as "todo" holes in the implementation. Inputs that reach these holes will cause the compiler to exit rather than silently proceed and either crash later or produce incorrect code.

This file is being added to enable continued development and progress on the lowering of OpenMP, OpenACC, etc.

[SouraVX]

Thanks for the patch!
NIT:
A more reasonable commit message could be:
[NFCI][flang] Introduce new helper TODO for further development
Or something similar(aligned to intent) :)

[schweitz]

In D88909#2314781, @SouraVX wrote:

Thanks for the patch!
NIT:
A more reasonable commit message could be:
[NFCI][flang] Introduce new helper TODO for further development
Or something similar(aligned to intent) :)

Good suggestion. I'll do that.

flang/include/flang/Lower/Todo.h

+//===-- Lower/Todo.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
+//
+//===----------------------------------------------------------------------===//
+//
+// Coding style: https://mlir.llvm.org/getting_started/DeveloperGuide/
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_LOWER_TODO_H
+#define FORTRAN_LOWER_TODO_H
+
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/raw_ostream.h"
+#include <cstdlib>
+
+// This is throw-away code used to mark areas of the code that have not yet been
+// developed.
+
+#undef TODO
+
+#ifdef NDEBUG
+
+// In a release build, just give a message and exit.
+#define TODO(ToDoMsg)                                                          \
+  do {                                                                         \
+    llvm::errs() << __FILE__ << ':' << __LINE__ << ": not yet implemented "    \
+                 << ToDoMsg << '\n';                                           \
+    std::exit(1);                                                              \
+  } while (false)
+
+#else
+
+#undef TODOQUOTE
+#define TODOQUOTE(X) #X
+
+// In a developer build, print a message and give a backtrace.
+#define TODO(ToDoMsg)                                                          \
+  do {                                                                         \
+    llvm::report_fatal_error(                                                  \
+        __FILE__ ":" TODOQUOTE(__LINE__) ": not yet implemented " ToDoMsg);    \
+  } while (false)
+
+#endif
+
+#endif // FORTRAN_LOWER_TODO_H