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

[Steven Wu via cfe-commits]

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

Do you want to avoid the extra space here?

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