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

[James Henderson via llvm-commits]

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@@ -834,6 +851,9 @@ def _executeShCmd(cmd, shenv, results, timeoutHelper):
         # Ensure args[0] is hashable.
         args[0] = expand_glob(args[0], cmd_shenv.cwd)[0]
 
+        # Expand all late substitutions
----------------
jh7370 wrote:

```suggestion
        # Expand all late substitutions.
```

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