[flang] Add the proposal document and rationale for the internal naming module...

[flang] Add the proposal document and rationale for the internal naming module that was previously added.

Summary:
This document describes how uniquing of internal names is done. This
name uniquing is done to support the constraints and invariants of the FIR
dialect of MLIR.

Reviewers: jeanPerier, mehdi_amini, DavidTruby, jdoerfert, sscalpone, kiranchandramohan

Reviewed By: jeanPerier, sscalpone, kiranchandramohan

Subscribers: tskeith, kiranchandramohan, rriddle, llvm-commits

Tags: #llvm, #flang

Differential Revision: https://reviews.llvm.org/D79089