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

[via llvm-commits]

jeanPerier wrote:

@Meinersbur, do you known if the GitHub CI bot is building flang-rt?

Looking at the GitHub checks on this PR, I see that the "Build and Test Windows" passed, and looking at the [build log](https://github.com/llvm/llvm-project/actions/runs/19711693190/job/56473772237), I see that flang-rt is enabled, but I do not really see its files being actually built.

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