[clang] [SafeBuffers] Check if unsafe-buffers* warnings are enabled earlier (PR #128603)

[Hans Wennborg via cfe-commits]

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@@ -0,0 +1,22 @@
+// RUN: %clang_cc1 -std=c++17 -Wno-all -Wunsafe-buffer-usage \
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zmodem wrote:

nit: No need for the line break?

https://github.com/llvm/llvm-project/pull/128603