[clang] [Clang] Fix assertion failure in SDiagsWriter when finish() is not called (PR #181712)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,8 @@
+// RUN: clang-check %s -- -Wdoes-not-exist --serialize-diagnostics %t.dia 2>&1 | FileCheck %s
+// RUN: ls %t.dia
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AaronBallman wrote:

What is the purpose to this run line?

(Perhaps the test is more clear if we serialize to `/dev/null` because we don't care about the output anyway?)

https://github.com/llvm/llvm-project/pull/181712