[Fortran] Fix/modify present() handling for assumed-shape optional (PR 94672)

[Fortran] Fix/modify present() handling for assumed-shape optional (PR 94672)

gcc/fortran/
2020-05-07  Tobias Burnus  <tobias@codesourcery.com>

	PR fortran/94672
	* trans.h (gfc_conv_expr_present): Add use_saved_decl=false argument.
	* trans-expr.c (gfc_conv_expr_present): Likewise; use DECL directly
	and only if use_saved_decl is true, use the actual PARAM_DECL arg (saved
	descriptor).
	* trans-array.c (gfc_trans_dummy_array_bias): Set local 'arg.0'
	variable to NULL if 'arg' is not present.
	* trans-openmp.c (gfc_omp_check_optional_argument): Simplify by checking
	'arg.0' instead of the true PARM_DECL.
	(gfc_omp_finish_clause): Remove setting 'arg.0' to NULL.

gcc/testsuite/
2020-05-07  Jakub Jelinek  <jakub@redhat.com>
	    Tobias Burnus  <tobias@codesourcery.com>

	PR fortran/94672
	* gfortran.dg/gomp/pr94672.f90: New.
	* gfortran.dg/missing_optional_dummy_6a.f90: Update scan-tree.