decl.c (gnat_to_gnu_entity): Constify a handful of local variables.

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Record_Type>: Constify
	a handful of local variables.
	For a derived untagged type that renames discriminants, change the type
	of the stored discriminants to a subtype with the bounds of the type
	of the visible discriminants.
	(build_subst_list): Rename local variable.

From-SVN: r199279