Implement the Named Return Value optimization.

Implement the Named Return Value optimization.

        * c-common.h (RETURN_NULLIFIED_P): New macro.
        * c-semantics.c (genrtl_return_stmt): Check it.
        * cp-tree.h (struct cp_language_function): Add x_return_value.
        (current_function_return_value): Now a macro.
        * decl.c: Don't define it.
        (define_label, finish_case_label): Don't clear it.
        (init_decl_processing): Don't register it with GC.
        * semantics.c (genrtl_finish_function): Don't check it for
        no_return_label.  Copy the RTL from the return value to
        current_function_return_value and walk, calling...
        (nullify_returns_r): ...this new fn.
        * typeck.c (check_return_expr): Set current_function_return_value.

        * expr.c (clear_storage): Set TREE_NOTHROW on the decl for memset.
        (emit_block_move): Likewise.

From-SVN: r43445