[clang] [flang] [flang] Implement -mcmodel flag (PR #95411)

[Andrzej Warzyński via cfe-commits]

https://github.com/banach-space approved this pull request.

The high-level stuff makes sense to me, thanks! I'm not familiar with `-mcmodel`, so can't tell for sure whether the tests are correct 😅  Ideally one more person would take a look - @tblah or @pawosm-arm ?

https://github.com/llvm/llvm-project/pull/95411