[clang] [Clang] Handle ?: operator in fold expression (PR #164019)

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Sirraide wrote:

> Could something else suppress `ExprError()`?

Returning `ExprError()` isn’t the same as issuing an error; we only consider compilation to have failed if an actual error _diagnostic_ is emitted.

https://github.com/llvm/llvm-project/pull/164019