PR c++/90449 - add -Winaccessible-base option.

PR c++/90449 - add -Winaccessible-base option.

       * doc/invoke.texi (Winaccessible-base): Document.

       * c.opt (Winaccessible-base): Added new option.

       * class.c (warn_about_ambiguous_bases): Changed name to:
       maybe_warn_about_inaccessible_bases.
       (maybe_warn_about_inaccessible_bases):  Implemented new
       Winaccessible-base warning option for both direct and virtual
       base warnings.
       (layout_class_type): Call to warn_about_ambiguous_bases changed to fit
       new name.

       	* g++.dg/warn/Winaccessible-base-1.C: New file.
       	* g++.dg/warn/Winaccessible-base-2.C: New file.
       	* g++.dg/warn/Winaccessible-virtual-base-1.C: New file.
       	* g++.dg/warn/Winaccessible-virtual-base-2.C: New file.

From-SVN: r272157