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

[Tom Eccles via cfe-commits]

https://github.com/tblah approved this pull request.

LGTM. Thanks for picking this up Mats.

Ideally, wait for @Thirumalai-Shaktivel to have another look before merging.

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