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

[Jon Roelofs via cfe-commits]

================
@@ -0,0 +1,14 @@
+// RUN: %clang %s -serialize-diagnostics %s.diag || true
----------------
jroelofs wrote:

```suggestion
// RUN: not %clang %s -serialize-diagnostics %s.diag
```

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