[clang-tools-extra] Add new tool: clang-read-diagnostics (PR #118522)

[Jon Roelofs via cfe-commits]

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@@ -0,0 +1,15 @@
+// REQUIRES: clang
+// RUN: not %clang %s -serialize-diagnostics %s.diag
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jroelofs wrote:

recommend putting the new file under `%t` instead of `%s`, so this doesn't pollute the source dir.

https://github.com/llvm/llvm-project/pull/118522