FileCheck not supporting large backrefs

[tsymalla]

If update_test_checks.py is ran on some lit test that contains a function with large signature and lots of metadata attached to it, all captures will be put in a single line.
For that, FileCheck can fail since it only supports up to 9 backrefs.

What is a feasible way to support such cases?

• Make update_test_checks.py count captures and split the lines containing the arguments into new lines
• Make FileCheck support more than 9 backrefs. The regex engine seems to include a \g<num> pattern, but it seems to not work here, \{num}, ${num} and $num etc. don't work as well (talking about a backref of around 40). Also, this can cause quite a performance regression.
• Use named capture groups with unique names to refer to the value of an earlier capture. I am not sure about the perf hit though.

Any ideas? In my opinion, this is nothing the Python update script should handle.