[clang] [libc++][lit] Allow overriding the executor for tests (PR #66545)

[Alexander Richardson via cfe-commits]

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@@ -134,3 +134,8 @@ ABI Affecting Changes
 
 Build System Changes
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+
+- The ``LIBCXX_EXECUTOR`` CMake variable has been deprecated. If you are relying on this, the new replacement
+  is either passing ``-Dexecutor=...`` to `llvm-lit`` or to make it persistent in the generated configuration
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arichardson wrote:

```suggestion
  is either passing ``-Dexecutor=...`` to ``llvm-lit`` or to make it persistent in the generated configuration
```

https://github.com/llvm/llvm-project/pull/66545