Fortran: Add diagnostic for F2018:C839 (TS29113:C535c)

Fortran: Add diagnostic for F2018:C839 (TS29113:C535c)

2021-10-08 Sandra Loosemore  <sandra@codesourcery.com>

	PR fortran/54753

gcc/fortran/
	* interface.c (gfc_compare_actual_formal): Add diagnostic
	for F2018:C839.  Refactor shared code and fix bugs with class
	array info lookup, and extend similar diagnostic from PR94110
	to also cover class types.

gcc/testsuite/
	* gfortran.dg/c-interop/c535c-1.f90: Rewrite and expand.
	* gfortran.dg/c-interop/c535c-2.f90: Remove xfails.
	* gfortran.dg/c-interop/c535c-3.f90: Likewise.
	* gfortran.dg/c-interop/c535c-4.f90: Likewise.
	* gfortran.dg/PR94110.f90: Extend to cover class types.