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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Worth using a syntax that matches something existing, like the Python one seems close? Or do we already diverge in a bunch of ways for the syntax in substitutions that there's no real consistency to be gained?

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