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

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.

Reviewers: jeanPerier, vjayathirtha-nv, sscalpone, jdoerfert, DavidTruby, kiranchandramohan

Reviewed By: sscalpone, kiranchandramohan

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D79177