c++: class NTTP and nested anon union [PR108566]

We were failing to come up with the name for the anonymous union.  It seems
like unfortunate redundancy, but the ABI does say that the name of an
anonymous union is its first named member.

	PR c++/108566

gcc/cp/ChangeLog:

	* mangle.cc (anon_aggr_naming_decl): New.
	(write_unqualified_name): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/anon6.C: New test.