* gcc-interface/decl.c (substitution_list): Rename to build_subst_list,
	remove unused parameter and simplify.
	(gnat_to_gnu_entity) <E_Record_Type>: Do not set TYPE_FIELDS.  Factor
	common predicate.  Rewrite loop for clarity.  Use GNU_TYPE directly
	as context for all discriminants.  Fix formatting nits.
	<E_Record_Subtype>: Add cosmetic 'break'.  Test Has_Discriminants
	before Discriminant_Constraint.  Adjust for above renaming.  Do not
	set GNU_TYPE more than once.
	(elaborate_entity): Test Has_Discriminants on the entity and use
	Implementation_Base_Type.
	(components_to_record): Rename component_list to gnat_component_list.
	Retrieve the _Parent field from the list.  Fix nits in comments.
	Clarify logic in loop.  Pass correct arguments to create_field_decl.

From-SVN: r148121