(duplicate_decls): Add new paramter different_binding_level.

Lots of changes to use new new parameter.
(pushdecl): Delete variable declared_global.  New variable
different_binding_level and code to set it.  Move extern/static
warning before duplicate_decls call.  Don't let global typedefs
conflict with nested extern declarations.  Move oldglobal test
inside code for setting IDENTIFIER_LIMBO_VALUE.
(lookup_name_current_level_global): Delete.

From-SVN: r10618