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

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

> Using `\g` for replacement looks good. https://www.regular-expressions.info/replacebackref.html

Thanks for the link. It's interesting that they mention the `\g<ref>` syntax only for referencing named groups, which we do not support.

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