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

[Tom Eccles via cfe-commits]

tblah 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.

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