[GH54588]Fix ItaniumMangler for NTTP unnamed unions w/ unnamed structs

As reported in https://github.com/llvm/llvm-project/issues/54588
and discussed in https://github.com/itanium-cxx-abi/cxx-abi/issues/139

We are supposed to do a DFS, pre-order, decl-order search for a name for
the union in this case. Prevoiusly we crashed because the IdentiferInfo
pointer was nullptr, so this makes sure we have a name in the cases
described by the ABI.

I added an llvm-unreachable to cover an unexpected case at the end of
the new function with information/reference to the ABI in case we come
up with some way to get back to here.

Differential Revision: https://reviews.llvm.org/D122820