[clang] [flang] [flang] Enable polymorphic lowering by default (PR #83285)

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jeanPerier wrote:

Thanks a lot for testing Tom!

I did not expect issues with gfortran tests. I will look into it before merging and I made a simpler first patch thet will be less noisy to revert in case of other issues that would break the build bots: https://github.com/llvm/llvm-project/pull/83308

https://github.com/llvm/llvm-project/pull/83285