Fortran: Set the vptr of a class typed result.

Fortran: Set the vptr of a class typed result.

	PR fortran/90076

gcc/fortran/ChangeLog:

	* trans-decl.cc (gfc_generate_function_code): Set vptr for
	results to declared class type.
	* trans-expr.cc (gfc_reset_vptr): Allow to provide the typespec
	instead of the expression.
	* trans.h (gfc_reset_vptr): Same.

gcc/testsuite/ChangeLog:

	* gfortran.dg/class_76.f90: Add declared vtab occurrence.
	* gfortran.dg/class_78.f90: New test.