[flang] [llvm] [flang] implement `split` for fortran 2023 (PR #161484)

[Tom Eccles via llvm-commits]

https://github.com/tblah commented:

Thank you for your work so far on this.

Please could you add unit tests for the actual runtime function (in flang-rt/unittesets/Runtime).

(I will be on holiday next week, to avoid blocking the PR I'm happy for the other reviewers to verify my comments are addressed and for you to merge with their approval)

https://github.com/llvm/llvm-project/pull/161484