[clang] [Clang] Do not try to transform invalid bindings (PR #125658)

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cor3ntin wrote:

> Oh actually, one thing to note is, how is this going to interact with #125394 since that just deletes the entire if statement that this change is part of?

Yeah, maybe it was not smart of me to work on that!
I suspect the fix with Jason's change is similar.

The other way to fix that is to move some of the checks in `ActOnCXXFoldExpr` to `BuildCXXFoldExpr` so that we can realize at this point we don't have packs.
But i think not transforming invalid binding deck is cleaner


https://github.com/llvm/llvm-project/pull/125658