Fortran: fix issues with class(*) assignment [PR114827]

Fortran: fix issues with class(*) assignment [PR114827]

gcc/fortran/ChangeLog:

	PR fortran/114827
	* trans-array.cc (gfc_alloc_allocatable_for_assignment): Take into
	account _len of unlimited polymorphic entities when calculating
	the effective element size for allocation size and array span.
	Set _len of lhs to _len of rhs.
	* trans-expr.cc (trans_class_assignment): Take into account _len
	of unlimited polymorphic entities for allocation size.

gcc/testsuite/ChangeLog:

	PR fortran/114827
	* gfortran.dg/asan/unlimited_polymorphic_34.f90: New test.