[clang] [flang] [flang] Add -fcomplex-arithmetic= option and select complex division algorithm (PR #146641)

[Tarun Prabhu via cfe-commits]

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

Aside from the slight change to the function names in `CommonArgs.cpp` and an optional tweak to the documentation text, this looks good to me. 

Thank you for PR and the clarifications to my comments.

https://github.com/llvm/llvm-project/pull/146641