[llvm] [lit] Add readfile substitution (PR #158441)

[James Henderson via llvm-commits]

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@@ -0,0 +1,6 @@
+## Tests that readfile works with absolute paths
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jh7370 wrote:

```suggestion
## Tests that readfile works with absolute paths.
```

https://github.com/llvm/llvm-project/pull/158441