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

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martinboehme wrote:

> Note there is a `BuildAnonymousStructUnionMemberReference`, I am not sure it solves your problem.

Thanks. The issue with this is that it requires an `IndirectFieldDecl` to call it, and that's hard to get at -- see also my response to @zygoloid.

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