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

[Erich Keane via cfe-commits]

erichkeane 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`.  

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