[clang] [Coverage][NFC] Avoid visiting non-unique `OpaqueValueExpr` (PR #88910)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Looks like automation didn't trigger for some reason... but quoting the automated message that's supposed to trigger:

> ⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
> Please turn off [Keep my email addresses private](https://github.com/settings/emails) setting in your account.
> See [LLVM Discourse](https://discourse.llvm.org/t/hidden-emails-on-github-should-we-do-something-about-it) for more information.

https://github.com/llvm/llvm-project/pull/88910