[flang] Update to fir::isUnlimitedPolymorphicType and fir::isPolymorphicType functions

This patch update the fir::isUnlimitedPolymorphicType function
to reflect the chosen design. It adds also a fir::isPolymorphicType
function.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D135143