[clang] [flang] [Flang] [Options] To be compatible with classic flang, add the -Mextend compilation option (PR #173833)

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> I think we should discuss in a discourse post before continuing with adding classic-flang options. 

I have added an [RFC on discourse](https://discourse.llvm.org/t/rfc-support-classic-flang-driver-options-in-flang/89380). We will also discuss this at the next [flang call](https://lanl-us.webex.com/lanl-us/j.php?MTID=mdce13c9bd55202e8071d8128fb953614) on Jan 14, 2026.



https://github.com/llvm/llvm-project/pull/173833