[flang-commits] [flang] [flang][Evaluate] Add `ExtractSubstring` to evaluate/tools.h (PR #81265)

[Valentin Clement バレンタイン クレメン via flang-commits]

clementval wrote:

> The use case is in an upcoming PR. I wanted to split things into smaller pieces and this looked general enough to exist on its own...

It's better to put it together with its use case or use stacked PR. 

https://github.com/llvm/llvm-project/pull/81265