Fortran: fix CLASS attribute handling [PR106856]

gcc/fortran/ChangeLog:

	PR fortran/106856
	* class.cc (gfc_build_class_symbol): Handle update of attributes of
	existing class container.
	(gfc_find_derived_vtab): Fix several memory leaks.
	(find_intrinsic_vtab): Ditto.
	* decl.cc (attr_decl1): Manage update of symbol attributes from
	CLASS attributes.
	* primary.cc (gfc_variable_attr): OPTIONAL shall not be taken or
	updated from the class container.
	* symbol.cc (free_old_symbol): Adjust management of symbol versions
	to not prematurely free array specs while working on the declation
	of CLASS variables.

gcc/testsuite/ChangeLog:

	PR fortran/106856
	* gfortran.dg/interface_41.f90: Remove dg-pattern from valid testcase.
	* gfortran.dg/class_74.f90: New test.
	* gfortran.dg/class_75.f90: New test.

Co-authored-by: Tobias Burnus <tobias@codesourcery.com>