[clang] [Clang][Sema] Fix malformed AST for anonymous class access in template. (PR #90842)

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martinboehme wrote:

> > This is the option I've decided to go with here. There's a slight wrinkle in that we create a LookupResult that claims we looked up the unnamed field for the anonymous class -- even though we would obviously never be able to look up an unnamed field. I think this is defensible and still better than the other alternatives, but I would welcome feedback on this from others who know the code better.
> 
> I actually prefer option#2. This doesn't really do what it should, so I think we should just make sure we materialize the temporary if necessary in `RebuildMemberExpr`.

Done. WDYT?

https://github.com/llvm/llvm-project/pull/90842