utils: add a helper class to lit for captured substitutions

utils/lit/lit/TestingConfig.py

@@ -152,3 +152,22 @@ def root(self):
         else:
             return self.parent.root
 
+class SubstituteCaptures:
+    """
+    Helper class to indicate that the substitutions contains backreferences.
+
+    This can be used as the following in lit.cfg to mark subsitutions as having
+    back-references::
+
+        config.substutions.append(('\b[^ ]*.cpp', SubstituteCaptures('\0.txt')))
+
+    """
+    def __init__(self, substitution):
+        self.substitution = substitution
+
+    def replace(self, pattern, replacement):
+        return self.substitution
+
+    def __str__(self):
+        return self.substitution
+