fortran: implement conditional expression for fortran 2023

fortran: implement conditional expression for fortran 2023

This patch adds support for conditional expressions in Fortran 2023 for a
limited set of types (logical, numerical), and also includes limited support
for conditional arguments without `.nil.` support.

gcc/fortran/ChangeLog:

	* dump-parse-tree.cc (show_expr): Add support for EXPR_CONDITIONAL.
	* expr.cc (gfc_get_conditional_expr): Add cond-expr constructor.
	(gfc_copy_expr, free_expr0, gfc_is_constant_expr,
	simplify_conditional, gfc_simplify_expr, gfc_check_init_expr,
	check_restricted, gfc_traverse_expr): Add support for EXPR_CONDITIONAL.
	* frontend-passes.cc (gfc_expr_walker): Ditto.
	* gfortran.h (enum expr_t): Add EXPR_CONDITIONAL.
	(gfc_get_operator_expr): Format fix.
	(gfc_get_conditional_expr): New decl.
	* matchexp.cc
	(match_conditional, match_primary): Parsing for EXPR_CONDITIONAL.
	* module.cc (mio_expr): Add support for EXPR_CONDITIONAL.
	* resolve.cc (resolve_conditional, gfc_resolve_expr): Ditto.
	* trans-array.cc (gfc_walk_conditional_expr, gfc_walk_subexpr): Ditto.
	* trans-expr.cc
	(gfc_conv_conditional_expr): Codegen for EXPR_CONDITIONAL.
	(gfc_apply_interface_mapping_to_expr, gfc_conv_expr,
	gfc_conv_expr_reference): Add support for EXPR_CONDITIONAL.

gcc/testsuite/ChangeLog:

	* gfortran.dg/conditional_1.f90: New test.
	* gfortran.dg/conditional_2.f90: New test.
	* gfortran.dg/conditional_3.f90: New test.
	* gfortran.dg/conditional_4.f90: New test.
	* gfortran.dg/conditional_5.f90: New test.
	* gfortran.dg/conditional_6.f90: New test.
	* gfortran.dg/conditional_7.f90: New test.
	* gfortran.dg/conditional_8.f90: New test.
	* gfortran.dg/conditional_9.f90: New test.