c++: Reject anonymous struct with bases

c++: Reject anonymous struct with bases

In discussion of jakub's patch for C++20 pointer-interconvertibility, it
came up that we allow anonymous structs to have bases, but don't do anything
usable with them.  Let's reject it.

The comment change is something I noticed while looking for the right place
to diagnose this: finish_struct_anon does not actually check for anything
invalid, so it shouldn't claim to.

gcc/cp/ChangeLog:

	* class.c (finish_struct_anon): Improve comment.
	* decl.c (fixup_anonymous_aggr): Reject anonymous struct
	with bases.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/anon-struct8.C: New test.