[flang] Add design document for debug info generation. (#86939)

This document discusses some options where the debug metadata can be
generated. It also goes through various language constructs and explains
how the debug metadata will look like for that construct and how we can
extract that information.

The real point of discussion is how and where to extract the information
about various language features to generate the debug metadata. The
structure of the metadata itself is mostly settled as that is dictated by
the DWARF and structure of LLVM IR metadata. The classic flang
and gfortran generate quite similar DWARF for the various language
constructs.

This document is based on what Kiran posted in
https://reviews.llvm.org/D138534.

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Co-authored-by: Tom Eccles <t@freedommail.info>
Co-authored-by: Kiran Chandramohan <kiranchandramohan@gmail.com>