c++: namespace-scoped friend in local class [PR69410]

do_friend was only considering class-qualified identifiers for the
qualified-id case, but we also need to skip local scope when there's an
explicit namespace scope.

	PR c++/69410

gcc/cp/ChangeLog:

	* friend.cc (do_friend): Handle namespace as scope argument.
	* decl.cc (grokdeclarator): Pass down in_namespace.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/friend24.C: New test.