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

[David Blaikie via llvm-commits]

dwblaikie wrote:

(We don't usually use "NFC" for functionality that's tested/testable - though I get where you're coming from, that this doesn't change the behavior of any LLVM program - but I'd probably leave off the NFC in this case (& I'm glad to see the test added - I think that's necessary for changes like this))

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