re PR c++/19317 (removing a temporary return value when we cannot)

        PR c++/19317
        Leave the return slot target in the MODIFY_EXPR rather than making
        it an argument, but only use it if the CALL_EXPR has a flag set.
        * tree.h (CALL_EXPR_HAS_RETURN_SLOT_ADDR): Rename to
        CALL_EXPR_RETURN_SLOT_OPT.
        * calls.c (expand_call): Adjust.
        * tree-inline.c (expand_call_inline): Adjust.
        * tree-pretty-print.c (dump_generic_node): Adjust.

        And set the flag as appropriate.
        * gimplify.c (gimplify_modify_expr_rhs): Set
        CALL_EXPR_HAS_RETURN_SLOT_ADDR where the LHS is obviously safe.
        * tree-nrv.c (execute_return_slot_opt): Set
        CALL_EXPR_HAS_RETURN_SLOT_ADDR based on escape analysis.
        * tree-pass.h: Declare pass_return_slot.
        * tree-optimize.c (init_tree_optimization_passes): Add it.

        * cp/semantics.c (simplify_aggr_init_expr): Use
        CALL_EXPR_RETURN_SLOT_OPT, not CALL_EXPR_HAS_RETURN_SLOT_ADDR.

From-SVN: r101269