[clang] [clang] Remove an incorrect assertion in ConstantFoldAttrs (PR #105789)

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Sirraide wrote:

> I guess it might make sense to create the `ConstantExpr` in any case, even for a `None` `APValue`?

The comment above `ConstantFoldAttrArgs` seems to suggest that that’s what’s supposed to happen if the expression isn’t dependent—I don’t know if there’s any code that depends on that, though.

https://github.com/llvm/llvm-project/pull/105789