c++/modules: import confict with builtin

c++/modules: import confict with builtin

27_io/headers/iostream/synopsis.cc was failing with modules because pushdecl
called update_binding for a redeclared import while passing the built-in
clog as "old".  This could also be fixed by discarding the builtin in
e.g. import_module_binding, but this seems straightforward.

gcc/cp/ChangeLog:

	* name-lookup.cc (pushdecl): Discard built-in sooner.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/builtin-9_a.C: New test.
	* g++.dg/modules/builtin-9_b.C: New test.