[clang] Stop double-diagnosing explicit convert operator in switch condition (PR #89142)

[Erich Keane via cfe-commits]

erichkeane wrote:

> Did you try to use `ExprDependence::Error`?

I've only ever seen that used in regards to the `RecoveryExpr`, so I think I am?  Also, `RecoveryExpr` doesn't really have a way of changing its dependence to anything but what it is.

https://github.com/llvm/llvm-project/pull/89142