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

[Tarun Prabhu via cfe-commits]

tarunprabhu wrote:

> A few of the tests in the gfortran-testsuite using polymorphism fail at runtime. I'm happy for this PR to be merged as it is and these bugs to be fixed later.

Once this is merged in, the polymorphism tests can be enabled together with the larger group of tests that I intend to enable in the next day or two. 

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