[flang] [llvm] [flang] implement show_descriptor intrinsic, a non-standard extension (PR #169137)

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

This needs to be documented in https://github.com/llvm/llvm-project/blob/main/flang/docs/Intrinsics.md

https://github.com/llvm/llvm-project/pull/169137