[llvm] [Support] Add `\{<ref>}` backreferences in Regex::sub() (PR #67220)

[David Blaikie via llvm-commits]

dwblaikie wrote:

I guess a workaround would be to add some empty braces to separate the matching number from the following digits? (`\3()42`)? Maybe that'd be sufficient, if a bit quirky? (I'm not totally opposed to this patch/feature addition either)

How's this compare to other regex libraries & what syntax they use for this situation?

https://github.com/llvm/llvm-project/pull/67220