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

AND the followups that fixed builds.

I attempted to get 'cute' and use llvm-cxxfilt to make the test look
nicer, but apparently some of the bots have a version of llvm-cxxfilt
that is not the in-tree one, so it fails to properly demangle the stuff.
I've disabled this "RUN" line.

This reverts commit 50186b63.