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

[Tom Eccles via cfe-commits]

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

LGTM. On a quick run this seems to pass everything I would expect it to in the gfortran test suite. When you get time please could you enable the polymorphic tests in the gfortran test suite and re-categorize the ones which now hit different TODOs.

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