[llvm] [FileCheck] Create test to highlight the "more than 9 back-references" issue. (PR #173501)

[Thomas Symalla via llvm-commits]

tsymalla wrote:

> This test looks like it passes based on the CI results?
> 
> 
> 
> It should probably be failing/marked xfail?

Yeah, XFail might be more suitable here. I'll update the test.

https://github.com/llvm/llvm-project/pull/173501