[flang] Add the header file for the conversion of FIR(MLIR) to LLVM-IR dialect

[schweitz]

Summary

This is the interface to the conversion pass that converts the FIR dialect (and standard dialects) of MLIR to the LLVM-IR dialect. The implementation of this bridge will be forthcoming in subsequent diffs.

The purpose of the code gen bridge is to take the IR, which will be expressed as a mix of higher-level Fortran constructs as expressed in the FIR dialect and the MLIR standard dialect, and convert the representation into the LLVM-IR dialect. At that point, the LLVM-IR dialect can be converted into LLVM IR (proper) by a standard MLIR pass.

[kiranchandramohan]

LGTM.
Is the plan to upstream the code for conversion to LLVM IR before the code for lowering from parse tree to FIR?

flang/include/flang/Optimizer/CodeGen/CodeGen.h

+//===-- Optimizer/CodeGen/CodeGen.h -- code generation ----------*- 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 OPTIMIZER_CODEGEN_CODEGEN_H
+#define OPTIMIZER_CODEGEN_CODEGEN_H
+
+#include <memory>
+
+namespace llvm {
+class raw_ostream;
+}
+namespace mlir {
+class Pass;
+}
+
+namespace fir {
+
+struct NameUniquer;
+
+/// Convert FIR to the LLVM IR dialect
+std::unique_ptr<mlir::Pass> createFIRToLLVMPass(NameUniquer &uniquer);
+
+/// Convert the LLVM IR dialect to LLVM-IR proper
+std::unique_ptr<mlir::Pass>
+createLLVMDialectToLLVMPass(llvm::raw_ostream &output);
+
+} // namespace fir
+
+#endif // OPTIMIZER_CODEGEN_CODEGEN_H