Make TREE_USED match the [basic.def.odr] concept for FUNCTION_DECL
	and VAR_DECL, so mark_used only has effect the first time.
	* decl2.c (mark_used): Just return if TREE_USED is already set.
	Don't set TREE_USED if cp_unevaluated_operand is set.
	(tree_used_ok): New fn.
	* init.c (build_offset_ref): Check it instead of TREE_USED.
	* call.c (build_call_a): Likewise.
	* cp-tree.h: Declare it.
	(DECL_NO_LINKAGE_CHECKED): No longer needed.
	(struct lang_decl_base): Remove no_linkage_checked bitfield.

From-SVN: r150807