Support `\g{n}`-style back references in `regcomp` as well by increasing
the limit from 9 to 20 and adding additional parsing. Update the limit
checks in FileCheck. The limit can theoretically be removed by
reallocating the regex-matchers internal arrays but I don't find a use
case for that as of now.
Update a test that now should pass when using more than 9
back-references.
Add a new test that tests for the error message explicitly..
636f34ea56