[llvm] [NFC][ADT] Introduce a test harness for StringRefTest (PR #105500)

[Rahul Joshi via llvm-commits]

jurahul wrote:

This is in preparation of adding reverse iterators to StringRef -> prep of using StringRef in SequenceToOffsetTable -> prep of using SequenceToOffsetTable in IntrinsicEmitter.

https://github.com/llvm/llvm-project/pull/105500