re PR c++/31187 (extern declaration of variable in anonymous namespace prevents use of its address as template argument)

        PR c++/31187                                                               
        * typeck.c (cp_type_readonly): New fn.                                         
        * cp-tree.h: Declare it.                                                       
        * decl.c (start_decl): Set implicit DECL_THIS_STATIC here.                     
        (cp_finish_decl): Not here.                                                    
                                                                                       
        * g++.dg/ext/visibility/anon3.C: New test.

From-SVN: r123432