[flang][NFC] Centralize fir.class addition in ConvertType

fir.class type is always needed for polymorphic and unlimited
polymorphic entities. Wrapping the element type with a fir.class
type was done in ConvertType for some case and else where in the
code for other. Centralize this in ConvertType when converting
from expr or symbol.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D143490