[llvm] [ADT][NFC] Make `getAutoSenseRadix` in `StringRef` global (PR #152503)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar approved this pull request.

Would be nice to add a unit test too now that this is a public method, but I won't block over this

https://github.com/llvm/llvm-project/pull/152503