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

[James Henderson via llvm-commits]

================
@@ -0,0 +1,8 @@
+## Tests that readfile works with two substitutions on the same line to ensure the
+## regular expressions are setup correctly.
----------------
jh7370 wrote:

```suggestion
## regular expressions are set up correctly.
```
("setup" -> noun; "set up" -> verb or adjective)
/end grammar pedantry

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