(duplicate_decls): Add new paramter different_binding_level.

(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.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@10618 138bc75d-0d04-0410-961f-82ee72b054a4