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

[via llvm-commits]

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@@ -720,6 +720,23 @@ def processRedirects(cmd, stdin_source, cmd_shenv, opened_files):
     return std_fds
 
 
+def _expandLateSubstitutions(arguments, cwd):
+    for i, arg in enumerate(arguments):
+        if not isinstance(arg, str):
+            continue
+
+        def _replaceReadFile(match):
+            filePath = match.group(1)
+            if not os.path.isabs(filePath):
+                filePath = os.path.join(cwd, filePath)
+            with open(filePath) as fileHandle:
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cmtice wrote:

And fail gracefully otherwise.

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