[clang] [Clang] Fix misleading diagnostic for pointer const type static data member (PR #223596)

[Mariya Podchishchaeva via cfe-commits]

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Fznamznon wrote:

Hmm... The pointer itself is not const though. @AaronBallman WDYT? Is that less confusing?

https://github.com/llvm/llvm-project/pull/223596