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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

I'm having second thoughts about leveraging recovery expressions as a side-band mechanism to de-duplicate diagnostics, because not modeling things properly in AST might backfire in the future. But I don't have anything better on my mind, so I don't want to block the progress of this PR.

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