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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Yeah, looks like Python's path probably started somewhere similar to us. Let's go with the python syntax, if you could - thanks!

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