[clang] [clang] Remove "unknown" from availability diags (PR #138610)

[Cyndy Ishida via cfe-commits]

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@@ -709,7 +711,7 @@ static AvailabilityResult CheckAvailability(ASTContext &Context,
             << EnvName << HintMessage;
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cyndyishida wrote:

no 😅 

https://github.com/llvm/llvm-project/pull/138610