PR c++/19542

    PR c++/19542
        * c-common.c (c_common_nodes_and_builtins): Create global null_node.
        (warn_strict_null_sentinel): Define.
        (check_function_sentinel): Check for null_node as valid sentinel too.
        * c-common.h (c_tree_index): Added CTI_NULL.
        (null_node) Define global_tree[CTI_NULL].
        (warn_strict_null_sentinel): Declare.
        * c-opts.c: (c_common_handle_option): Handle -Wstrict-null-sentinel.
        * c.opt: (Wstrict-null-sentinel): New C++ option.
        * doc/invoke.texi (C++ Options): Document -Wstrict-null-sentinel.

        * cp-tree.h (cp_tree_index): Remove CPTI_NULL, to be defined in C
        common frontend.
        (null_node): Remove.
        * lex.c (cxx_init): Move null_node initialisation to C common frontend.

        * g++.dg/warn/sentinel.C: New testcase for __null sentinels added.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@99091 138bc75d-0d04-0410-961f-82ee72b054a4