[flang] [llvm] [flang/flang-rt] Implement show_descriptor intrinsic, a non-standard extension. (PR #170389)

[Slava Zakharin via llvm-commits]

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

Thank you, Valery! LGTM (with two minor style remarks)

https://github.com/llvm/llvm-project/pull/170389